I set the door flat on a pair of spacers, and apply the clamps across the ends. Just snug the clamps up for now. It’s your chance to check final alignment of the rails with the ends of stiles. At this point, you can use a square to check the frame. It’s a good idea to measure across the middle of the door, too. The panel may cause the stiles to bow out slightly. So you may need to add a clamp to draw the stiles straight. At last, tighten down the clamps. In a few hours, you can take the clamps off and be confident that the door is ready to go.