This will be a short lesson. It’s all about adding “noise.”
(You are free to use textures from online, but I personally don’t like them.)
“Noise” is a filter that I’ve seen most programs have. It’s usually under the filter tab/section.
The filter should be on top of the base of the design. (The colored in part.)
Just adjust the filter to your liking, then lower the opacity so it looks good.
Here’s what it should somewhat look like:
(Personally, I just feel it adds some type of texture so it doesn’t look so flat and 2d)