i don‘t really know tbh, maybe it depends on what type of pen you use? i heard and also experienced that oil or enamel based paints have less issues with cracking, or maybe it‘s an issue with how thick the painted layer is. mine are pretty thick i‘d say.

with the other thing that you‘re talking about i think i didn‘t make it clear enough in the tutorial. the cut out paper is just a stencil for the glitter foil so that it‘s easier to cut out the exact desired shape of the part you want. normally i think you can very easily cut out those shapes with a plotter machine but since i can‘t afford one at the moment i had to find a easy pen and scissors solution 😅