Simple Steps 1 - 2 - 3
Import photo of texture from camera roll
Resize and move the texture image to the desired position over your artwork
From the layer pallet - Click on the N next to the image and select desired blend mode. You can also slide the opacity level for desired till it looks good to you.
Texture makes artwork pop
Texture kicks artwork up a notch. It makes it more visually interesting and more for your eye to take in.
These steps are for the Procreate app, but can be repeated in Photoshop as well.
The next time you find interesting texture, take a photo of it and try to layer it onto some of your artwork.