The standard stencil/decal workflow in Blender sucks. Shrinkwrap, parenting, projections.... ugh! 😐🔫
This addon fixes that! Automated material based projection!

1. Select the target
2. Shift + Ctrl + D (for Stamp Master) and pick a stamp
3. Set guided coordinates (tip: you can also do fading with the mouse wheel)
4. Apply

This bad boy has a fully integrated layering system and noise based fading. If your image has transparency (or even if not), then you're good to go
(sorry for the .gif quality here, there is a file limit size apparently)

I would recommend watching the tutorial for a full breakdown: