I look in the cupboards and the freezer and 'fridge, and see what we have each night, or every few nights, if there are leftovers to be eaten.
I have never planned out an entire week at once--tried it a few times--it did not work for me. Either there would be nothing leftover if I had counted on that for another meal, or there would be way too many leftovers, and foul up the rest of the week's plan...(at the time, I didn't have very much freezer space).
I just play it by ear, (or by palate) and "see what we feel like tonight." I think I was raised that way. My mom never did a menu plan, either, and back then, we had a small neighborhood, local grocer, local butcher shop, etc...within 2 blocks walk. Since mom did not drive, she went to the store every day, as there was no way she could carry a week's supply of food, and we did not have a freezer at all.
I recall her asking my dad now & then on weekends, "what do you want for dinner tonight?" and he'd as often as not reply, "I don't know--see what they've got when you get to the store."
So, I never have done advance meal planning....we just keep a fair amount of assorted food on hand...and go from there.
(My eldest daughter, on the other hand, plans her week's meals practically down to the last kernel of corn, with nothing extra for unexpected company, or anything else. She'd have to come to our house to survive if there were a disaster of any kind!)