Here's what I do and it works really really well. Gives perfect coffee every time, but your milage may vary so don't be afraid to experiment.

Grind to filter coffe size or maybe a bit finer. Don't go to espresso level or it'll just clog the brewer and burn the coffee. It is generally not super sensitive to grind size so don't worry too much. If in doubt go corser.

Always, always weigh the coffe! I found that they are actually quite sensitive to the amount of coffe you put in. Ever since a started weighing the beans I've never had any issues with it. If you cannot get a nice consistent flow through the entire brew and it begins to sputter too early you most likely filled it too much. Rather than fiddle with the grind size, you should instead fill it a little less next time. I use 12-13g for the 2 cup, and 28-30g for the 6 cup version. If I go outside that range it won't brew nicely. It's that sensitive.

Finally use hot water. This first of speed up the brew but also makes it easier to control the pressure inside. So pour hot/freshly boiled water in, assemble, and put on a low to medium heat. When the coffee starts to come through turn it down to low heat to get a nice, slow and consistent flow. For the 2 cup version it's about 15-30 seconds and for the big 6 cup version it's about 40-90 seconds. I can't remember exactly so don't worry too much as long as it's consistent and nice and slow.

Stir and pour the coffee immediately. Otherwise it'll slowly burn and turn bitter in the hot brewer.

With this method i always get amazing coffee out of this little machine and i low it so much.

I hope this was helpful.

Edit: I see from other comments that you use a blade grinder. This is fine, and I've used one for years when I was a student and it worked just fine as well. You can grind pretty fine with that for the mokka pot, but again, if on doubt go corser. More importantly is to avoid clumps. Blade grinders tends to make clumps so try to stir them out a bit with a needle/scewer or a very thin fork. Clumps cause channeling and easily makes it sputter and all the water goes through too fast since it cannot build the pressure needed to make a consistent flow. On the other hand if it sputters and goes really slow then it's clogged instead.