I really like how Marco of
https://diystuff.nl/tradfri/tradfri-zigbee-light-link-module/ reverse engineered the IKEA tradfri module (or rather I really like the end-result


I asked him once how he did it, but sadly it's mostly just manual work

Without fancy tools, what I do, I load up various photo's in separate layers (I use the GIMP) and then use the opacity slider in the layer toolbox, pretty much as to what Dave suggested. A snapshot of 50% looks like

.
When adding the layers, getting orientation, rotation etc can all be adjusted (requires a beefy CPU too :p) so yes getting everything ideaal is best when taking the photo's, but you can fix quite a lot actually with the '3D transform' I think, as there are anchor points that can be used for alignment (vias for example).
Even adding an X-Ray layer can be useful

but then you really need to do the outline thing first (hadn't done that bit yet ...) and again, doing fades on multiple layers in the gimp is easy enough to highlight the things you care about ...
Following that I trace out (either through some image tricks with outline selections n stuff or even manually) the traces in separate colors, to try to get the same result Marco has. Mine where never as pretty

My feeble attempt in the attachment ...
Anyway, some food for thought ...
(sorry for the oversized pic there)