Honestly depends what's wrong with it and why you're rebuilding. Just like an engine rebuild.
A pro transmission builder will do stuff like check all the metal for microscopic fractures, deburr all the gears, etc. and give you back a transmission that's better than new.
But you can also just dive in and slap in new synchros in your home garage.
I'm sure you've already youtubed it, but for an example of a pro rebuild:
https://www.youtube.com/watch?v=5OWI7kz5hCo
vs. a home rebuild. Note that he still used a press! I don't think you can do it without a press. Also note that both he and the pro ended up having to order parts from the UAE, so it's not a weekend job.
https://www.youtube.com/watch?v=opJ7trkonYU
If you're good at 3d shape puzzles a manual transmission is straightforward.