I know of one technique but its quite tedious and pretty annoying. Works best with low poly.
spilt the screen
press shift + F10 on one screen
press f on the 3d screen and select a group of faces
load a blank white texture on the other screen
basically, just select each of the faces, press u to insert it into the blank texture then just gotta connect each polygon as if you unwrapped it yourself…tedious? Hell yeah…
After you end up with an unwrapped wireframe of your model, press the paint button and trace out the wireframe in the uv texture screen (other half). Save the image file, save the blend.
Open the jpeg (texture with the wireframe outline) in a paint program and paint away. When you’ve done the colours and whatever, save it, then convert to greyscale, save that as another file
to use the grey scale, open the blender file go to materials, add texture, goto texture window and load the grayscale image
Go back to materials window and the switch the “Orco” button to “UV”.
Got to the far right of the materials window and turn off “Col” and turn on “Nor” and adjust the value underneath.
There’s probably a way easier…er…way but I could find it, at least not in Blender.
heres the bump test