A lot of people have asked me how I make my backgrounds look like AC style, and I think much of it comes down to patterns. You can easily make your own patterns by using screenshots, or the samples I've included here. Let me know if any of this doesn't make sense and/or if you have any questions!
for the custom Gouache "Chalk" brush I use.
And I didn't exactly explain masking, but you can quick mask any selection within the Layers box.