Hi,
Last year I wrote up everything I know about making one-page RPGs at Skeleton Code Machine. It's been quite popular, and I've even taught a class using it at a public library. Covers game design theory (e.g. the kinds of fun), different types of one-page games (e.g. journaling), layout, publishing, and has some tools to help you get started like the Theme-O-Matic. Each chapter has exercises so you can follow along and end with a finished game.
You can read the Make Your Own One-Page RPG series for free here: https://www.skeletoncodemachine.com/p/one-page-rpg
Or, if you'd prefer a formatted PDF, you can grab that here: https://exeuntpress.itch.io/one-page-rpg