I've shaved blind in the shower every day for years. I just run my hand along my face directly in front of the razor as I shave to feel what the razor is about to run over before it actually gets there. For the sideburn area I put my finger at the edge of what I want to shave and just shave upwards until I bump against my finger. For the lips I just kinda curl them inward to make them as flush with my face as possible. I've memorized the contours of my face at this point.
I've shaved blind in the shower every day for years. I just run my hand along my face directly in front of the razor as I shave to feel what the razor is about to run over before it actually gets there. For the sideburn area I put my finger at the edge of what I want to shave and just shave upwards until I bump against my finger. For the lips I just kinda curl them inward to make them as flush with my face as possible. I've memorized the contours of my face at this point.