I built my bunkie from the ground up, but the thing that impresses my guests most is how the ends of the deck boards follow the shape of the adjacent rock. It looks amazing, but the technique is simple: scribing involves using a compass to transfer irregular contours to your workpiece, then cutting the shape so the piece fits snugly.
Position a board so that it butts up to the rock. Run the pointy end of the compass against the rock so that the pencil end marks the same shape on the end of your board. Cut the shape with a jig saw, install, and repeat for each deck board. Be sure to account for the material you’ll be removing when cutting the overall length of the board. If the board you’re cutting won’t fit into the space initially, scribe onto a scrap of cardboard, and use that as a pattern.
I used the same scribing technique when I had to make custom-fitted deck footings on uneven bedrock. I built plywood box forms and scribed the bottom edge of the forms to the rock. In this case, temporarily shim the forms with some scrap wood so that the tops are level. My compass was too small to span the distance required so I cobbled together a length of dowel (you can also use a scrap piece of wood) with a felt-tipped marker taped to one end. As the end of the dowel traced the surface of the rock, the marker made the same shape on the side of the form.