I rebuilt mine this winter. I ordered the Badshoe videos. They are literally step by step video instructions. Starts out with three transmissions on a bench. Tells you how to i.d. which one you have (early or late c4, or c6). After you i.d. yours, it is pretty easy to follow. The only tool some may not have is the gauge to measure end play as there are different sizes of thrust washers to put you end play into spec. He shows how this all works in the video.
I also ordered a custom rebuild kit from Ken at Badshoe. He only sent me the parts I needed and the exact thrust washers I needed. It was all quality, most of it Raybestos.
He also answered all my questions and responded to every email I sent asking questions.
Honestly, if you have a torque wrench, basic tools and can rent or borrow the gauge to measure end play, it was very simple.
PS: my slipping, leaking c4 now shifts like butter...and no leaks!
1966 Mustang Convertible