This is more of a tip than a guideline, but a lot of ideas will come when you don't expect them, because once you start working on a problem, your brain can be working on the solution and then you might passively come up with the idea somewhere when you you're doing something else and are not ready for it.
For me that happens a lot in the shower. I do a lot of foggy glass sketches in the shower while my mind is focussed on shampooing for some reason. When I used to drive to work, it used to happen in the car. Or in New York it happened in the subway.
This is something that is common for a lot of designers and artists. Austin Kleon's book Steal Like an Artist talks about the artist David Hockney, who had suits and pants tailored to have pockets to hold sketchbooks so he was always ready when an idea came.
I carry paper in all sizes from sketchbooks to the little Field Notes books and the pocket Moleskine notebooks, and Levenger business card-sized cards that I keep in my wallet.
I also work on real and imagined projects to practice. While I’m on the subway, out at a cafe, anywhere, I practice my shapes and lettering, and take lots of notes to myself. I also take lots of screenshots and keep a swap file of things to steal.
Doodles are also important. I think of the doodling I do as my bonsai. It’s something I do repeatedly in my sketchbook with no real purpose, but sometimes the random doodles lead to real ideas.