well, i guess you could call it a tutorial. i'm just going to tell you how i made some awesome christmas gifts. and yes, they're great gifts, but i couldn't tell you how to do it before christmas, because i was making them for people who might see my blog!
get tiles. they're like, 10 cents for the cheap-o ones at Lowe's. it's pretty standard to have 4 per set.
find cork, and cut barely smaller than the coasters. my coasters were 4.25" squares, so i cut 4" squares for the cork. oh, and the cork came from target. i'm sure you could get it at a michael's or hobby lobby, but it was a fraction of the cost for 12" squares that i cut into many squares for coasters.
hot glue the coaster to the back of the tile.
cut your photos or fabric to match the tile. again, i did a little smaller, like a 4" square, so there was a slight edge where the tile starts to angle differently.
put a thin layer of mod podge (or similar decoupaging substance) on the tile, place your photo (or fabric) where you want it to go, and press it so there aren't many bubbles. for the photos, it's important to press them or else you'll get a giant bubble right in the middle. layer with mod podge. i use a foam brush and try to not have many streaks in it.
let them dry. when they're mostly dry, do another layer of mod podge, going in the other direction from your first application.
after they've dried the whole way (usually i wait until the next morning to do this step), spray them with a clear matte spray. this helps them to be waterproof-- you don't want all that hard work wearing off with the first glass set on it, do you??-- and it also makes them look professional and really finished.
congrats! you've just made a beautiful set of coasters for a birthday or christmas present! it's neat to do something personal. my friend used her own photos and gave it to family members. i used (probably copyrighted) photos from a Las Vegas book to match my daddy's LV bar. you can use anything! be creative.