On a notebook, make a drawing. Make sure the lines are black !!
Then take a picture (make sure to photograph it in a lighted place, preferably in natural light)
You can also (if necessary) adjust the brightness / contrast.
Finally, set the layer to [multiply] mode
Put the basic colors below the line. Then add the shadows and the brightness as you usually do.
💡 If you want to select a color from the canvas, I advise you to hide the layer of the line and to select your color.
To make the drawing more interesting, I advise you to put a colored background (create a new layer, move it below the last layer, draw the background)
You can also add white decorations
Personally I like to add grain in another app.
HERE IS! I hope this little tutorial helped you ^^