Make it up! It's your world, so put your own spin on it. If someone complains, BS a reason behind it and connect it to some lore about a hero that changed or fucked something up and since then they've always done it that way.
Some things to consider:

• operating funds. Should they have a minimum of gp to set up? Should their be a minimum per month operating cost for admin staff? Taxes by city?
• location. Should they need a guild hall? Do they have to buy the property or rent?
• buyin from residents. Do they need support from residents to do it? Do they need to take care of some sidequests to get support of a few cantankerous councilman?
• make up a weird third category. Do they need to throw a feast? Do they need to decorate their hall with the heads of monsters they killed? Does the local temple offer a blessing (with benefits for members) if they do some task?

Make up some requirements to be able to create a guild (maybe there is a guilds-guild where you need to register). Requirements could be:

1. A fee to register your guild. This could be very expensive. Maybe they can find an investor but need to do a favour for them first.
2. You need a location inside Waterdeep. Also expensive, maybe they can get one via a quest. Or if they have Trollskull Manor, they could make some changes to the building to make it suitable as a guild house.
3. You need at least 15 members. They need to try to recruit people.
4. ...

Also, maybe some other guilds are not happy with this (they think there are already too many guilds, or the guild is too similar to their guild) and they try to sabotage the players.

It's a good opportunity to introduce NPCs you may want to use later. Perhaps the open or masked lords, or a bureaucrat who will be helpful later.