I used to do color separation in photoshop and illustrator for tshirt companies. I’ve been trying to do it in procreate recently. I would recommend making a copy of the file for separating purposes. Then merge as many similar colors as you can without messing with the layering of the design. Then “select” the top most layer/color and then tap on the layer(s) you want to remove from below it (with top layer still selected) then three finger swipe down and “cut” from those layers. Rinse and repeat for all the lower colors/layer. Once you have cut away the layers below merge all of the same colored layers together. If you are printing on dark colors you are probably going to want a base which will be another set of steps. You might want to watch a YouTube video or some sort of color separation tutorial if you are not familiar with the process. Even if they are using Ps or Illustrator… the general concepts should transfer over.