You truly don't need to be fancy about keeping a sketchbook.
Get a small one that is portable and keep it with you all the time.
It can be a place to freely doodle and play.
You'll be amazed at how your drawing skills improve
simply by taking a minute here, a minute there.
At the airport.
Staying awake at church.
Waiting for food at a restaurant.
Insomnia late at night (drawn from a photo).
Just DO it!