It's non-trivial.
Have a good clean image of the board and both (or all) layers. Then identify the componets and try to untangle the connections. It's often useful to identify the "main" part -- an op-amp, or a microcontroller, etc, then draw everything around it in a way that looks sensible.
You need to be very careful that you haven't missed a connection. It may be useful to colour them in as you go.
After you've drawn it once, see if you can untangle it and make it look nice.
Then check for obvious errors and refer back to the PCB to see if you made a mistake.