how do i season my cast iron pan?

i’ve had this pan for years, i think it used to have more of a coating but lately it’s looked really bare. i don’t wash it with dish soap. i know you’re supposed to coat it with olive oil and put it over heat for a while to let it build up its seasoning, but it doesn’t seem to be working. and when i cooked an omelette in it the other day it left this gross black residue all over the bottom of the omelette. any advice?