It depends really on how much dismantling you need to do. It's not a job for the fainthearted, mainly because of the various odd one off tools you may need plus the difficulty of getting the bits.
A bearing should be easy enough to get, if you go to a bearing specialist (surprisingly, you can find one in most towns) with the old bearing in your hand so they can match it up.
Old Haynes manuals used to feature a gearbox strip, but it's been dropped from the modern manuals as so few people actually do it.
Read through carefully, and see what tools you need, wether you have them all or need to get anything special: gearboxes tend to have the odd massive nut which you'll need a big socket for, and you'll also need some good circlip pliers (inside and outside) and pullers as well................................And then think "sod this" and take it to a transmission specialist to fix, or buy an exchange box. If you have it out of the car that's a good slice of the labour costs saved.