Since Zac Gorman asked,
1) Scan your paper or other piece of whatever you intend to use as “texture”.
2) Adjust levels etc. as necessary. Get it looking nice and even (assuming this is the effect you want).
3) Run the “High Pass” Photoshop Filter on that flattened image. Find it under FILTER > OTHER > HIGH PASS
3b) Tweak High Pass parameters as necessary. Get it just right and you win a stuffed Finn from Adventure Time.
4) Layer that High-Passed image above the layers you want to affect.
5) Set that layer’s Blend Mode to “Overlay”.
Voila. Hopefully the High Pass filter sucked out the essence of the texture you want to use. (If not, figure out another way.) Since the High Pass-ed image is mostly 50% grey, setting it to “Overlay” doesn’t mess with your image’s colour too much because I guess that’s how Overlay works. Shrug?