Just look at a photo and cut the fabric correctly, then put a zipper in front
Depends on which one. If you want a TOS pattern, they have one in Franz Josef's Starfleet Technical Manual. Other costume patterns can be found at fabric stores or online. Star Trek fan clubs are a good source, they usually have people who are into costuming.
Links to find your local Starfleet Command/Starfleet Int'l chapter:
Roddenberry.com has several styles: