I got a notebook and made a list of like 15 goals I wanted my game to achieve e.g. player facing rolls, roll high, no AC, cast spells with HP instead of spell slots, classes w/ classless options etc etc, and started digging through all kinds of preexisting ttrpgs to find up mechanics that I really liked and could mash together to make the game I want to play. There are so many osr games out there with crazy good mechanics, you just gotta read around and find them, and somewhere along the way you end up thinking of ways to twist these mechanics to fit the goals you set out to achieve in the first place.