"What helps me is masking tape. I do one side of the line at a time. If the line is straight then I take a long piece of masking tape and attach it on the upper side of the body line on a spot on the car that is good and then pull the tape and attach it to a good spot on the car on the other side of my work area. This shows me where the line is supposed to be. With the tape in place I block the lower area only. I block with a long board parallel to the tape bringing the board up to the tape line and then off the car into space. When I get it looking good I put another piece of tape on the area I just sanded just next to the first tape.
Now remove the first tape and block the top with the long board parallel to the tape again. Blocking down to the tape but keep the board on the top surface and don’t let it break over the line. You cross the line but don’t let the block roll into the lower surface. Let it come off the car into space. If you are careful you won’t hit your tape and you will get a nice crisp line. You may need to round the edge some but be careful, it flattens down fast.
Others may do this differently but the tape works for me."