Normally, drywall is hung horizontally, but due to its size and shape, you can really hang any direction in the shaft because you're only going to need 4 pieces typically and they won't butt up to other pieces on the same wall. (Here's a tip: I will usually cut the first piece about 1/4 of an inch shorter than the measurement. It will be covered by the adjecent drywall and make less rework when fitting.)
How you orient this will determine if you have to have any butt joints or finish each wall in one piece (which you want to do). Using a drywall square (because it's longer) measure from one corner the distance of the bottom opening measurement, mark it. That edge is your bottom. Also note that you are using the proper side of the drywall. Don't use the rougher side, it's usually darker as well. Measure up from the bottom right side of the drywall, this is the long point measurement. You should be on the end of the drywall and therefore should have a square edge.
Use the drywall square to go up from the far mark and measure up. This is the short point that you previously measured. Now connect the dots from the end of the long point, to the end of the short point. The way you cut this piece will be determined by how it is oriented. Experience says to cut the short side all the way through, even past the measurement, break the drywall off and then cut the angled part off. This is how drywall is hung and it takes a little experience to learn the proper ways to cut pieces for certain situations. If you've never hung drywall before, this won't be enough for you to learn so just try to get the piece cut in the right shape.
Once it's cut, use a drywall plane to smooth out any roughness on the edges, especially the top. Now slide it up there and see if it fits. Hopefully the top looks good. This is the most important part unless you plan on putting some sort of wood casing up there to make it look pretty.
It will probably need a little tweaking here or there, planing or trimming. It's normal, don't feel bad. You'll get better at it with time. When all is well, either screw or nail it in place with the appropriate fasteners. I recommend every 8 inches on the edges and every 12 inches or so in the middle.