The thing you will do most -and almost never can overdo- is to wash. I use alcohol to clean my bodies (without any kind of grease/silicone in it), both before masking and before painting. And before painting. And before painting. Did i say before painting?
I find alcohol easier to use as i just use a little rag to wash with, and don't have to soak the body in water as when using the dish soap-method. This will maybe ruin the masking, and getting water where you don't want it. The first thing i would have done was to use the scotch brite, then -as you say- ; mild dish soap, dry body, tape off design and window mask,
(remember to wash/clean at this point) lay dark color on first, then lighter colors, then back entire car shell with white backing
Looking forward to see the body finished, do post some pics off it when it has got some colours on it!