Hi Koolii,
No problem but may I know which part of the rules you didn't understand? Maybe if it's about the game design itself there's only 1 rule. Your rules or game text itself needs to fit a bookmark paper size.
Also, it needs to be a playable game by itself whether it is a character generator, item generator, story generator, prompts generator, or simplified D&D guidelines. It needs to fit in a bookmark. You can design it as a single page or double page.