First: The tools you'll need can be purchased at any Home Depot, Lowes, Northern Tool, Ace Hardware of similar place of business.
Here's a list of what you'll need. Links to said tools are provided.
24" straight edge. A large carpenter's square will work as well, and is actually cheaper. Use a hacksaw to shorten it, and you've got two!
Single cut 6" mill file. Cut off the tang so it will lay perfectly flat.
A depth gauge or a set of cheap calipers
Various grits of sandpaper, I prefer to use 220, 320, 400, 480, 520, 580, 600 and the four "0" grades of steel wool (0,00,000,0000).
Let's get started, shall we?
First, remove the strings and have the guitar rested firmly on a table, workbench, or the like. Support the neck with something sturdy but soft at approx. a 15% angle. Remove all hardware that could possibly come off (TOM bridges and tailpieces especially).
Loosen the truss rod, and using the straightedge you just purchased, get the neck to as little relief as possible. If you can't get it perfectly straight, at least try and get it into the low thousandths of an inch. Now, normally I work a TINY bit of relief into the frets so that I can have a stronger, straighter neck but still achieve maximum playability. We'll save that for another time
Using a slightly damp cloth (I prefer the blue shop paper towels) give the board a good 10-15 second wipe down. This will remove any oil or grime buildup on the frets that could hinder proper measuring, and give you a clean work area. After you've done all this, using the depth gauge or calipers, mark the exact height of your fret crown above the board.
Take a sharpie marker (I prefer the chisel tipped black ones), mark the very top of the crown of the fret down the entire board. Go slow, and be careful. If you slip, it's a PITA to get sharpie out of a light colored rosewood board, and lemme tell ya about those satin maple Fender necks. Just because we're on a budget, don't mean we ain't gonna do things right.
Now, using our modified single cut mill file, use very light (and I mean LIGHT) pressure, run the file lengthwise down the board. Notice how the Sharpie's disappearing? Your goal is here, to make the Sharpie disappear in an even fashion. So, starting at the edge (I usually start at the treble and work right to left, but whatever is most comfortable for you), remove the very tops of the frets. Remember to take it slow and don't rush it. Now, once you get to the middle of the board, move to the bass side and work your way inward. What this does is maintain the radius, and minimizes the ability to flatten out the tops of the frets.
Do that twice, or until any worn down spots have disappeared. Try not to drop the top of the frets down too low, because you're still going to have to do the crowning by hand using sandpaper, and well it's hard to do when your frets are too low. I know, I know, but we're on a budget, and not exactly trained repairmen (except for those of us at the Luthier's Workshop. Enough name dropping and rubbing it in ).
Now, before we go all willy nilly with some sandpaper, we need to tape off the board. I suggest that while you're out at your favorite tool emporium, you pick up some 3/4" blue painters or drafters tape. If you'd like to save yourself the trouble of cutting up itty bitty pieces of tape to fit between the frets in the higher register, you can check your local drafting store, college bookstore that stocks school needed art supplies, Micheal's, Hobby Lobby, or the like for some 1/4" and 1/8" masking tape. It's a great time saver, but not "needed" for what we're doing here. Don't forget to run some tape along the fingerboard edges, unless you're wanting to roll the edges of the board while we're doing this step.