You'll need to remove the floor over the transmission, which should be self explanatory. Once you can see the top of your transmission you'll notice the top plate bolted down right on top of the transmission which contains your shifting mechanism. Simply remove the top plate bolts, then carefully lift the plate so you can see the position of the shift forks, because you're going to need to get the forks back into the same position as when you removed them.
You'll probably need to take the top plate to a transmission shop to get the detent ball and spring replaced unless you can find those parts yourself. The detent ball and spring are what holds the transmission in gear. Sometimes the spring breaks and other times the detent ball becomes so ward that it just falls out of place.
During reassembly it should be immediately obvious if the shift forks are not in the right position because the transmission won't shift correctly and you can feel it.
After those little parts are replaced, and if you can get the shift forks back in the proper positions, bolt the top cap back onto the transmission and replace the floor.
In theory that should do it. It's never clear following someone else's instructions. I haven't worked on that specific transmission but manual transmissions are pretty similar.