To make a full-size drawing, I clamp a framing square to a sheet of plywood and scribe along both legs: one line for the ceiling and the other for the wall. I place a piece of the crown against both legs, tight inside the square. Then I scribe all the way around the molding.
When you’re making cuts for either inside or outside corners, the cut along the flat, vertical section at the top of the molding should always run perpendicular to the top edge.
I find the distance from the bottom of the molding to the ceiling, then I determine the length of my returns.
I mark the bottom of the crown profile on the wall using a scrap that I cut exactly 3 15/16 inches long.
I cut a strip of wood exactly 100 inches long, butt it into one corner, and mark the end on the wall. Then I measure from the mark to the other corner with a metal ruler, adding 100 to that measurement to find the total length.
Transferring measurements to a floppy piece of molding can be challenging, especially when measuring from an end that’s cut at an angle. When measuring from the long point, I hold the tape in place with a spring clamp.
To measure from the short point of a cut, I clamp a Speed Square across the piece and hook the tape over the square.
A simple site-assembled rig holds up the far end of the material for me. I clamp a horizontal 2x4 a couple of feet long in a Rockwell Jaw Horse and attach a vertical strip of plywood about 8 inches wide and about 80 inches long. I slide the strip up to the correct height and clamp it to the 2x4 with a one-hand bar clamp.
My work station is an old door on a couple of sawhorses. I'll put a piece of plywood on top to set my sliding compund miter saw on. Note the simple material support on the left side.