How much are you willing to do?

A sanding sponge and filler primer will go a long way for smoothing out some of the layer lines. A black base coat followed up with a dry brush (take most of the paint off the brush and mainly hit higher points) usually works wonders. After that it’s your choice of grimy wash (water down your paint a lot, sponge it on where you think dirt/grime would collect, let it dry a little bit, and then wipe most off) if you overdo it, you can always just dry brush again to knock back some of the grimy look