I just do it, usually while thinking about other stuff.
So, for me if I start thinking about other stuff very much while doing something like brushing my teeth, I stop doing the something. Like, literally I’ll snap out of a chain of thought that I mostly don’t remember (‘why am I thinking about harvesting cranberries? I need to get to work!’) and I’ll realize I’ve just spent a minute or two standing still in the bathroom holding my toothbrush and toothpaste staring at nothing in particular.
If I start thinking about something mid-process I’ll end up missing part. The entire process requires attention. I have to mentally label 12 areas, A-L, then count 10 seconds for each area, every single time, A 0123456789, B0123456789, C0123456789, etc.