I have index cards that I file in a kids school supply pencil box.
I used to use a just a plain spiral notebook to do meal planning - like to have them listed out each week to control repeating too much. I updated it to one of those bullet journals recently.
I also use Pintrest a lot for recipes which I love for finding recipes, but it's often hard to COOK by pintrest so if I try one that I like, I'll write it up on a index card for the next time.
I cook for the family almost every night, so I have to be organized with it.

-Susan