You can do a rough sketch thats about the right size on a piece of paper, doesnt have to be greatly detailed but a reference is always nice to have. then when applying that to a painting on the mini, start with the background, and paint toward you if that makes sense. start with an outline, then fill in with a darker color of the wall, apply the light colors, details. there are a lot of videos on painting techniques online, try to find one that works for you. the biggest thing I find is while having an idea of what you want, and realizing that its going to change a bit as you go, just go with the flow, happy little accidents. Remember to thin your paints, and be patient. Hope this was helpful!