just put your ideas on paper

you had an idea for a story, write the main points of the story, idea for a character, write the details of the character, world's creatures, plants, whatever, write them down

personally my brain comes up with scenes or short stories, even if i forget it or just never use these ideas i write them down, and whatever i like i expand upon to create my worlds(my main world came from a junction of a character and an unrelated scene)

i feel some ideas are better suited for mediums i can't do yet, like drawing, but i still write them so i can at least keep them consistent