I've learned a few lessons from the recent layered projects I've worked on, like Wonderland, Delight, and Coming Home. I will share them with you in hopes that this will save you from a few struggles and some paper.
1. Lights for the foreground and darks for the background
This applies to many artistic mediums, and you've probably heard it before. But it is something to remember. Light colors are best used for objects in the foreground or the main subject of a piece. The opposite is true for dark colors. They should be used for less unusual shapes and those appearing in the background.
In addition, make sure that you have enough contrast between the colors. For example, on the image on the right side the colors are too similar in tone. This makes them compete for attention and the artwork looks flat. The image on the left has more balance of lights and darks and contrast amongst the colors.
2. Don't shy away from bright colors
The same lesson for light and dark applies to bright and muted colors. The reason I bring this up again is that I was surprised to find that the colors that I shy away from (like neon pinks and greens) are the ones that I'm finding work really well on these pieces. They stand out in a good way and also add a cheerful feeling.
3. Test before you glue
Wait until you've cut out all the layers and seen what they look like one over the other before deciding whether they work or not. I did not do this; I glued everything together and ended up with a piece that didn't quite work. It looks drabby and sad.
4. Use the glue you feel comfortable using
I've recommended using a needle tip applicator, and sometimes, I've mentioned spray adhesive. I realize the latter isn't for everyone. It takes some practice. The suggestions I offer in the PDFs are just that, suggestions. Do what you think works with your abilities, needs, and frankly, patience - lol. I'd say this also applies to any other tips I offer. Make these projects comfortable for you and how you like to create them. Just have fun!
I hope these help! If you have any questions, and as always, please feel free to reach out.