Josh!
1st - Get two absolutly plane pieces of (hard)wood.
2nd - Get the sort of tape to have underneathe the carpet that sticks to both sides.
3rd - Cut the wood as broad as your tape and something like 4-6" long.
4th - Get 40 grid and 100 grid sanding paper, and glue one to each woodblock with the tape.
Now you have half of the perfect tool for flattening the surface, the other half of the tool is your patience!!!
You can flatten the glueside of your overlay material on it, too; just clamp the woodblocks to your vise, and sand the overlay on it!
Have a look at my overlay-build-along:
http://www.primitivearcher.com/smf/index.php/topic,1729.0.htmlHope that helps!