One of our own (In this thread even) has put together a comprehensive reverse engineering method here:
https://www.instructables.com/id/How-to-reverse-engineer-a-schematic-from-a-circuit/
In princple it's easy in concept especially for a single layer, but in practice it's not. For a single sided board, you need to take a photo of the component and solder side and then reverse the solder side image and add some transparency to the component side.
Then build a three layer image of solder side and the translucent component side and a translucent drawing layer. They all have to be scaled and aligned.
Now draw the connections on the translucent top layer.