Here's the "how-to"
1) Sweep your wall clean of dust and cobwebs with a broom. If you want a white base coat, paint the whole wall with your base. I just left the off-white primer that was already there. If you paint a base color. Let it dry 2 days before taping.
2) Using 5" bits of painters' masking tape (I prefer the green "Frog Tape"), figure out roughly how you want the horizontal stripes to be spaced on your wall. As you are playing with the spacing, step back a bit and make sure you are satisfied with the placement on the stripes on the wall--the center of the wall isn't always the best, and be sure to consider cabinets, tables etc in determining how high you want it on the wall. I ended up going with with 4" stripes of blue on either side of an 8" center stripe. The white in between the stripes is just the width of the tape-- about 1 3/4".
3) Once you have a pattern with your 5" strips where you want them and spaced how you want them, measure from the ceiling down to the top of each strip of tape and write the measurements down, numbering them 1, 2, 3, 4 etc. NOTE--it's crucial to make these measurements from the ceiling. Garage floor are not plumb, so if you measure from the floor, your lines will not be straight.
4) Taking your measurements, measure and mark them at each corner of the garage, and in the middle of the walls. Use a pencil and label each mark 1, 2, 3, or 4 for quick reference.
5) Use a chalk line to snap lines across the walls. For long walls use the center mark and make two snaps instead of one long one.
6) Determine where you want your Y on the wall. Set up a projector and laptop and, using the highest resolution Y logo you can find online, project the logo onto the wall. Use the chalk lines as a reference for placement of the Y. We had to use a ladder for the projector, and ended up putting the Y ~1" above the center of the lines.
7) With the projector on, use painters' tape to tape the borders of the blue (tape should be on the white portions. This is slow work as doing the curves will require a lot of 1" or so bits of tape since it doesn't do curves well. If you're skilled, you could hand paint the curved edges, but I'm not that guy. Make sure every edge that will touch blue paint is smooth--especially important where one side of the tape touches the logo, and the other touches your stripes.
8) Once the logo is taped, tape your stripes, following your chalk lines. Once everything is taped, go over and rub all the tape firmly to get maximum seal.
9) If you want super crisp lines, where blue paint will be touching the tape, first paint with your white base coat. This way any bleeding under the tape will be base coat colored and won't show. If you don't care too much about super crisp lines, skip on to 10.
10) Use a brush to paint your corners, painting away from the tape to minimize bleeding. After brush work is done, break out the roller and hit it. Repeat with a second coat. You can pull your tape as soon as you are done rolling the second coat and you're confident your paint won't run down the wall.
11) The next day, once your paint has dried, wipe the wall with a damp rag to eliminate excess chalk from your lines. Use a soft pencil eraser to take care of any of your measurement marks the paint didn't cover.
12) Gloat to friends and family.
As for color, I just picked the closest blue I could find in the Kilz Pro-X 300 series paint at Home Depot. They don't have many bases, so you can't go super dark to get to the official BYU blue, but it's a cheap high quality paint. I used ~1/2 a gallon to do the whole project, and 1 gallon of semi-gloss cost $20. You'll want to use semi-gloss or glossy in the garage so you can wipe it down good. Most Lowe's and Home Depots in Utah County will have an official BYU blue, but you'll have to go to a Behr or something with darker bases to get to that blue, so you'll add about $10 to get the darker color.
I used nearly two rolls of Frog Tape, which was another $16, so project total was <$40, two hours prep, and two 20 minute paint sessions. If anyone in Utah County wants to give it a go, let me know when you have the tape and projector in hand and I'll come over and help you set it up. Cost of my services = 12 pack of Mtn Dew.