Mom/wife here: The foil shaver I use to touch up my own fades and my husband/kids heads is this: https://wahlpro.com/products/5-star/5-star-shaver/

Depends on what part of your head/face you're working on, but i have found a good amount of success going against the grain in a clockwise circular motion, where I want it to be the baldest. Imagine scrubbing the hairs off with your foil shaver, sort of.

If your hair isn't short enough, it won't fit in the tiny holes in the foil, so it won't work for touch ups unless you shave it down with an edger first. I have this combo: https://smile.amazon.com/ANDIS-Professional-Barber-Combo-CL-66325/dp/B008YKNLWM

You don't need that combo, but that's what I'm working with. The foil thing is dope though, my hair grows back super fast and it helps my fades last a lot longer.

Hope this helps: