How do I build this body line

Guide coat the bondo, lay masking tape down following one side of the line, block the other side to the tape, pull tape up you'll see the line on the still guide coated side, and lay a masking line down on the side you just sanded, then block guide coated side to the tape. Alternate back and forth with each coat of plastic. it can make for a very sharp line. Get it built up, then you go back and soften/round the edge. Hope that makes sense. I like to work the upper surfaces flat mask it out then work the recesses. I also work an area as one piece. Each coat goes over the whole area. Hard to get them nice if you go back dab a little bondo here and there. You'll waffle up the rest of the area working it that way. Little blocks, cut down Paint sticks, sandpaper wrapped around a cardboard tube etc. Use stuff similar to the contours your trying to match.