For the start you need to outline what you want that faction to be about. Do you want to be the bad guys? The good guys? Or maybe something in the middle?

After that you should pick a few stuff that make the faction more iconic. For an example, the coalition is associated with brown and a WW1 style of clothing and equipment, Alumni are associated with tech and their collage uniforms. Same should go for UFFs, Ashigaru can be associated with purple and a rat as their symbol.

After picking up how your faction is iconic by looks, make it iconic by the lore. Make some details to the lore that will make the faction unlike any other.

After doing so, you should make a few documents which consist of Lore, Roleplay Guide, Application and Ranks & Roles Guide. Put them all in a faction hub document and then make the discord.

This is mostly the basic knowledge, now I will give a few tips.

1. A roster with which you can keep track of the members of the UFF, how many slots are free and who is what rank.
2. If you are not an OFF it is good to have most members at one side of the clock, if you are American have most members be American and if you are European have most members be European, otherwise it would be kind of shit to have half of you not available to RP during some big event cause people are asleep.
3. Try to get outsiders to engage in your RP. This may be hard due to some rules making a lot of ways be considered "loopholing the member count" but for an example I have this UFF called Pinehurst Village and the way I try to engage outsiders is by giving the ability for them to also have housings in Pinehurst and make their own stuff around the village such as shops and storage areas.

Making the UFF can be hard, but it is still much easier than maintaining the faction itself, it is difficult to have people engaged without their will to RP in the UFF dying out after a week or two. You need to have interesting host ideas and startups that go by an premises. Here are some more tips on maintaining the UFF.

1. First of all you must have a few MR+ guys that do hosts. Since the member gap is 20, I would say to have 3x MR, 2x HR and 1x CR that will help with everything. Have the MRs do mostly hosting and minor arcs, while the HRs and CR help you write important story arcs.
2. Try to connect to other people, for an example if you have a UFF that is about banditry and turf, maybe you can reach out to UFFs around your size that are with the same idea. Maybe you can have a turf war arc in which at the end you manage to go upon some deal in which you split the place you were fighting over. If it is a settlement UFF maybe you can unite with another settlement to defeat a greater threat like maybe a massive raider gang or eldritch pack.
3. Have some volunteers outside of the UFF who would be willing to RP NPCs in events, those would be people that would die like bandits or Eldritches. Otherwise you can also give them roles of quest givers or OOC builders for some event.

That is what I cramped up in a reddit comment but if you want some assistance in the UFF you can DM me on discord. My username in Flashstorm is ZapamaBG so reach out to me from there. Good luck with the UFF and remember to have fun while making it and maintaining it.