Place two stepladders along the first wall so that they are roughly the same distance apart as the length of the Sheetrock panels. Take one end of the first Sheetrock panel and have another person take the opposite end, and climb onto the ladders. Hold the Sheetrock panel up against the ceiling, so that the length of the panel is perpendicular to the joists.
Drive screws through either edge of the Sheetrock and into the most central joist that you can reach from your position on the ladder, using a screw gun. Position the screws so that they are roughly ½ inch in from the edges and sink them into the Sheetrock so that it dips in 1/8 inch. Move outward from that central joist toward the end of the Sheetrock panel, driving screws into each joist that you pass out to your end of the panel. When you reach the outermost joist, drive screws through the panel every 8 inches.
Go to the other end of the Sheetrock being held up by the other person and repeat the process, beginning at the innermost joist you can reach and moving out to the end of the panel. Once both ends have been fastened in place, move a ladder beneath the center of the Sheetrock and attach the panel to the joists you couldn't reach.
Move down the wall and place the next Sheetrock panel at the end of the first. If the Sheetrock is too long to fit as a full piece, measure the distance from the end of the first panel to the wall, transfer the measurement to the Sheetrock and cut the panel to size by slicing across the back of the Sheetrock with a utility knife and then across the back of the Sheetrock along the same line to fully separate the panel. Screw the panel into place.
Begin each subsequent row of sheetrock with the leftover piece of panel that you cut to size, so that your seams stagger automatically. Place each panel flush against the row before it and use the same screwing method to secure each panel to the joists. When you reach the final row, measure the distance between the last row of full panels to the wall, and cut the panels vertically to fit into the leftover space.