There is a guide here: https://stellaris.paradoxwikis.com/Star_Trek_New_Civilisations/Empires/United_Federation_of_Planets

Originally posted by GlitterBoyPilot:

Im missing something, have restarted 5 times and still cant form the UFP. Prime Universe is on. Milky Way Large. I form an alliance with the Vulcans, and cant get cohesion up, alliance dissolved. Form with Andorians, same issue, Tellarites, same issue.

Ive changed the leadership to Random which give a small bonus. But there seems to be no events that actually help form the Federation. I have all DLCs. no other mods installed and active.

Ive initiated peace conferences to raise trust. Sent my pitiful number of envoys to improve relations. Used an Envoy to the Alliance. Nothing works.

HELP!

You need to be in an alliance with at least two of them. It can sometimes be tricky to get the Andorians and Vulcans on board. However you get the various events in the play through. ultimately you will need to side with the Andorians and expose the Vulcans placing weapons on the spiritual planet. Yes, your Vulcan rep takes a hit from this, but the Andorian increases. However if your Vulcan one was high enough before hand, it can survive it.

I'd suggest you use the envoys and improve with both of them. For me the Telerites can be a little more tricky and I would often leave those for last.

Then you just need to wait for the events to trigger and make the right choices. If you side with the Vulcans good luck getting the Andorians on board.

Well, the UFP itself won't form by random event. You need to run the agenda when it becomes available.