From here I usually clarify details with cleaner linework on a separate layer; If I'm feeling frisky I might also create a guide layer to figure out perspective and stuff. Usually, I just jump right in and lock it down, so I can move on to the next idea buzzing around in my brain. Gotta love ADHD.

Add some finer details with a Sharpie pen, spritz some Dullcote over the top, and you're good to go.

I've found a quick way to give a weathered look is to lay down base colors with acrylic paint markers, add some shading with water-based markers, and then blend it with a light-colored alcohol-based marker, which gives a grimy, aged quality to everything.

You might also try checking out the online app Womp(www.womp.com). It takes a different approach to 3D that might feel more intuitive.

There are some good tutorials on Youtube that helped me a lot, like videos by Dave Reed and SouthernGFX.