As Tony said, rare earth magnets and paper is more accurate than the bright light method.

To summarize the method I use:

- After tracing the top, make a duplicate on tracing paper, cut out the soundhole, bridge, and fretboard extension, and tape to the top with low tack drafting tape.

- Place one magnet inside the body and the other on the top, and allow them to come together. By moving the inside magnet around, the edges of all braces can be found by marking where the outside magnet sits.

- Make a couple stops along each side of the X, the bridge plate, tone bars, fingers, UTB, and popsicle brace.

- Once done, remove the tracing from the top of the guitar and transfer the brace locations to the original body tracing.

- Repeat the process for the back braces (although they can usually be pretty well located through the soundhole with ruler, etc.

- Some careful work with ruler (scallop locations, etc.) and a ruler stub with a stop block (brace heights) will allow profiles to be worked.