I’m considering writing a short tutorial on using render_targets for post-processing, but it wont be ready in the nearest future, so tell me if you need any hints on how to apply your post-process shaders to the game. There are some basics posted in this thread, they might help you for the start.
But for the shaders itself - yeah, its mostly shadertoy, google-fu and knowledge of basic math. Old film is basically sepia (easy). vignette (easy, harder if you want image distortion), grain (easy), and moving lines (probably kinda hard to make them look good). All of this you can google as a separate shader and then combine them in one (this will reduce a bit of drawcalls, which is good).
P.S. Hey, Defold team, how about adding a feature where you can dynamically add/remove postprocess shaders without adding a quad on the scene and a bunch of code into renderscript?