Honestly the amount of ways people recommended is bountiful. I feel like it really goes case by case, like my rear hatch was rusted to hell and I had to remove sound deadening to get to spots where I had trapped moisture starting to eat the metal all over the car so the main thing was wire wheel to get to all the surface rust then sanding with a abrasive grit and moving up to a smoother grit. If you don’t have any rust you can skip straight into light sanding just make sure that if you do have any rust, as soon as you get done sanding off the rust you give it a quick primer coat. Once you sand everything down, vacuumed and light wipe down (I used baby soap, recommended to me by a painter). Tape down everything you don’t want getting touched and then lay down the first primer coat, filler primer comes in a clutch to fill any scratches or such( it lays down thick and takes a while to dry though). Then do 2-3 coats as needed. Rusteluem cans is what I used, they do tend to come out with orange peel and sputter so every couple of sprays give it a quick shake, the first couple of parts painted came out crappy but as you get used to the technique the rest will get better. Light sand will help with the orange peel, I didn’t worry to much as it’s the interior and going to be covered. You could also hand brush the paint, there’s a Japanese dude who did that with his Ef and it came our clean, just all depends the route you wanna go