How to cause dynamic screen distortions

I apologize if this has been answered somewhere as I couldn't find it from looking around. I'm trying to make something where I can cause effects like skewing, bowing in/out or slanting things on the screen, but also say I want two areas skewed at different angles, oh maybe I want another part of the screen slanted over here all of a sudden, et cetera.

I looked into shaders to try and figure out how to do this but I admit I really don't understand how to work with them too well, and it seems like the shaders I've run into don't let me change how much they apply their effect dynamically (I may be mistaken of course).

I know this is a rather vague and odd request but any help or guidance in what I should look into would be great. I'd also be happy to provide more explanation.