Hang the bottom sheet first. If you can't figure the angle then use something to mark where 4' in height will hit the ceiling (you could slide the sheet of drywall in and use that as a gauge). Mark the floor where the sheet hits the ceiling. Measure the distance from the bottom corner to your mark, use that measurement to go from the top corner of the sheet of drywall back, mark that point. Then use a chalk line or a straight edge to go from the bottom corner to the point on the top you marked- that should be your angle.

Remember, you want to keep your drywall about 3/4" off the floor, don't let it touch the concrete. If you cut it right you can use the cutoff piece as a template for your other side and should be able to use the actual piece for your top on the other side.

Sorry if the rambles, it's actually easier then it sounds. Good luck

Thank you so much for taking the time to respond! I will give this a go, and let you know if I have any trouble.