Not exactly explaining why Socialism is Democratic but tell them about how many Democratically Elected countries Amerikkka couped.
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you could start with the small scale stuff like that measuring tape guy in shenzhen (look it up) or if they aren't convinced by the local level stuff, point to the 1937 soviet consitution or the cuban family code (best examples I know of widescale consultation on national policy)
Simply by explaining that under socialism, capital does not give you political power.
I think the best way to approach this topic is to begin by breaking down the very concept of democracy. I think you should do your best to illustrate the fact that democracy is an spectrum and that. Even feudal Europe was in part "democratic", even if it was extremely undemocratic.
Once there, i would simply point out at the facts, like the abysmal two party system, or the abhorrent approval rating of almost every single usa president in the past 100 years, or the fact that even though it has been the wealthiest nation on earth for 100 years TWENTY PERCENT OF ITS POPULATION CANNOT READ. And of course at the CPC approval rating and the wonders it had provided for the Chinese people.
And i would close it up with something like "Do you think that the two party dictatorship of the usa, serves its people better than the CPC?".
I think this is a pretty good sketch of an argument.
I am not very good at this, when I say our democracy is not democratic they said I sound insane
I would say a simple litmus test with "democracy" for somebody individualism-brained is, "Are your interests being represented? When you have a complaint with how things are run, what is it like bringing it up and will it get listened to?" If their interests generally are getting represented and the process for getting complaints dealt with is robust, in a liberal "democracy", it's likely that either you're talking to someone who is among the exploiting classes and so isn't likely to be moved by your criticisms of it, or they are thinking of trivial window dressing complaints rather than important needs.
In other words, getting to the heart of "what is the point of a democracy?" If it does not generally represent the people's critical needs and seek to empower them, then what is the point of it? Growing up in a liberal "democracy" can at times have an odd sort of glorification of process that goes on. "You have a right to vote, on X day you write down your vote, on Y day representatives get elected." But what does this mean if it doesn't translate to positive outcomes for people?
A misguided conclusion might be "well I guess democracy is bad then" and indeed some people living in liberal "democracies" end up drawing that conclusion. But that's part two and that gets into who owns the means of production and where power derives from and for whose interests; why it is that the so-called democratic systems of capitalism don't bend toward representing the needs of regular people. If they think liberal "democracy" is good and they're among the more exploited classes, something is going wrong in how they think about it and what their standards are.
Talk about the World Bank, IMF and WTO, and NATO. Those are the actual institutions that make up the near global Yankian empire and dictate terms undemocratically to the vast majority of the worlds population. Ime one never gets anywhere comparing defending the USSR et al until ones shown the global nature of the western system and that any limited democracy in the North is still at the expense of the South
Losurdo wrote a piece on "Herrenvolk democracy," ie democratization was allowed in limited degrees for workers in the imperial core, while exerting absolute dictatorship on the global south. "Democracy for the master race."
If democracy is social but not economic, it isn't democratic. I can vote for the guy who will tax my boss less, but I can't vote to elect a new boss or increase my pay or decrease my rent. My boss, landlord, and the political class all enshrine their own power over me the more they amass wealth. When their vote has more weight than my vote, it's the same dynamic as pretending to let your pet choose something. The illusion of control obscures a fundamentally uneven relationship.
Under socialism, my vote improves my life. It isn't a pressure valve to prevent revolution, but a means of achieving it across all the spheres of my life.
I invoke the communes of Venezuela, when you break them down it's clear to anyone that the process is more democratic than electoralism in the west. It's an actionable example because they exist despite the country struggling.
Of course if you're invoking Venezuela to an antisocialist you have a whole other bag of worms to deal with.
Although people here have provided fantastic resources, I think the question may need to be approached a bit differently.
Intent is immediately relavent.
For one, if you're going to argue someone, it's important to know if this person is an associate, friend, colleague, or stranger, and if they are even open to changing their mind in the span of a single conversation. If they aren't, then it's not really worth arguing beyond the benifet of any on lookers or to simply plant seeds that hopefully sprout later.
With that said, and assuming you're talkng to someone in goid faith, the next thing I think it's important to understand is:
why is this person arguing with me?
A lot of the time, when people assert that western liberal democracy is more democratic than Marxism-Leninism, they aren't actually concerned with the debate about which structure of governance better facilitates rule by the people. What they are often doing is challenging the idea that ML states will work to their benifet and not restrict them unnecessarily. They see how Western liberal democracy does not restrict their ability to complain but nothing gets done, and they are fearful that if an ML state works against their benifet, then it'll leave them with less then they had before.
It's why they usually use hypotheticals to explain their point or ask questions. They don't fundamentally understand that an ML state is structured in a way that privileges them as workers.
You have to be able to expand the other persons view of how they participate in soceity
So as well as explaining how the Vanguard party works in a place like China, Cuba, Vietnam, etc, (which comrades here have provided resources on) it's important to discuss other areas of popular control in these projects.
For example, in China, every large enterprise has a union and worker council attached to it. This means that your average worker doesn't just have representation in their local political scene, but at their workplace as well.
It's also good, in my opinion, to explain how Chinese state ownership of large industries actually works. As in, it's a gradual level of nationalization where as an industry expands, higher levels of governance from local to national, link that industry to the communities they represent.
Lastly, it's good to just bring up the fact that ML states consistently follows the will of its people. China especially, due to its largely unrestricted access to the global economy, consistently makes decisions that benifet its people when it has every oppurtnity not to, like the US. At some point it just becomes ludicrous to believe that a country's leadership is able to male the correct decisions 99% of the time without any input of its populace.
Democracy can only really be measured by how effectively it achieves the needs, representation and approval for those it represents. Western liberal democracies don't do this. They are only democratic from the perspective of capitlists.
Those who control the commanding heights of industry have a powerful hold on the foundations of society, and this has cascading impacts on the rest of society. The state is also not outside and above the realm of production and distribution, but thoroughly enmeshed within it, and therefore must be the representative of the ruling class. Socialism allows the working people to determine the direction society heads in, and direct the social surplus as they see fit, rather than be appropriated by private parties.
That's extremely simplified, perhaps overly so, but this is generally where I start.
https://en.wikipedia.org/wiki/Campaign_finance_in_the_United_States
https://en.wikipedia.org/wiki/Minimum_wage_in_the_United_States
https://en.wikipedia.org/wiki/Wealth_inequality_in_the_United_States
The sum of money required to win political office continues to increase every cycle, while the minimum wage has continued to decline in real terms over the same period. Inequality of wealth is continuously getting worse as well. The critical thing to consider is the class of the person you are trying to convince. If they still believe they can achieve significant wealth in their lifetime, you have a very slim chance to getting through to them.
i am from germany
The only general limitation on party expenses is the ability to raise funds from private contributors. Due to a supreme court ruling (see below in History), public subsidies to a specific party may not exceed the "self-generated revenue", i.e. the total amount raised from signed-up members and other donors ("relative maximum", "relative Obergrenze"). Thus, a party that is unable to collect enough contributions from private sources will see its public subsidy reduced automatically[15] and be unable to make all the expenditure that it would like to.
This is a really advantageous arrangement for capital. If a party doesn't have significant private funds raised, they are unable to gather public funding.

CGTN's interactive presentation on how the Communist Party of China is organized.
This is roughly how every communist council system of state governance is organized.
Another way to look at it is look at the class background of communist leadership across the history socialist states. How often are they from privileged socio-economic backgrounds? Only about 9 presidents grew up in what would be concidered working class backgrounds.
23 American Presidents came from either high socio-economic status backgrounds, politically prominent families, or both. 26 of them were lawyers, and about 22 of them were career politicians.
What was Stalin? A peasant that got expelled from priest school. Mao? A librarian, I believe. Khrushchev? A metal worker. Up and down the the political chain, the working class compose the bulk of the political body that makes up the Soviet socialist system of governance
I often describe it has democracy extended to the economic sector