Line Up and Install the Lower Piece
Remove the supporting screws you installed earlier. Then lean the bottom piece of drywall against the wall, and transfer the stud lines onto it. You might want to use a speed square or a level and a straightedge to help you draw the lines.
Jack up the bottom of the lower piece of drywall so it butts closely against the upper piece. If you don't have a helper, use a drywall jack (Image 1) to lift the lower piece. It's okay if there's a slight gap between the top and bottom pieces.
Use drywall screws to begin securing the lower piece to the wall studs. Drive the first screws through the beveled edge at the top of the drywall (Image 2).