Create a meeting zone and choose “l” to set a location. Choose the option to create a new location and then choose the option for a new temple. You will then see a list of all of the deities (and non-deities) worshipped in your fortress along with the number of worshippers of each. Ignore these. In the same list you should also find entries for every religion represented in your fortress and the number of members that they have. The one you want is probably the religion with the highest number of worshippers that you haven’t already made a temple for. Select that religion from the list.
Afterwords, to appoint the priest, from the game’s main menu, select “l” for the locations menu, select the entry for the temple and there should be an option to appoint a priest which will bring up a list of the religions members for you to choose from. Also, don’t forget to use the “r” key to set the restrictions appropriately, and remember that you still need to furnish the room (smoothing, engraving, statues if you’re looking for FUN, etc) so that it has a certain minimum value before it counts as a temple for the purpose of satisfying the agreement. Actually, IIRC, I believe the game won’t let you choose a priest until the temple “qualifies” as an actual temple. Also, IIRC, I think the list of of deities and religions might also say “Agreed to build a temple” for each religion that you agreed to build a temple for.
Hope that helps!
Edit:
Also note that, in the list of deities and religions, the entries for religions are colored differently from the entries for deities and also list the deity that the religion worships.