Clean it. Scuff it. Clean it. Grease remover. Bed liner 2 coats. I rolled mine. Used Gator I think. Get extra rollers as they dissolve quickly. Will need at least 10 rollers for 2 coats. Don't have to be like the ones that come in kit. Just foam roller.
They all suck. I bought the more expensive LMC version. They took some work to make fit. Do all you bending before primer and paint. Fit them with the necessary shims and bend them where they need to be bent, remember how many and size and location of shims then remove door and primer paint.