1. ƒirst, rustle yourself up a sweatshirt or t-shirt you want to paint on. I got lucky, and found this black one for $1 on sale at the Rugged Warehouse, but I assume a Goodwill will do you just as well. American Apparel shirts are great for this purpose.
2. Make a stencil. I found the Saints logo's I used by simply typing in "Saints logo" on Google.
For example:

I printed it out, and traced it onto a thicker paper. An old paper shopping bag would do the trick nicely if you haven't happened to have an internship where your only job is to pass our fliers, and you got bored after 75 or so and hid the last 25 at home. Here are my stencils:
In this picture, they've already been painted on.
3. Lay out your sweatshirt and pin your stencils down flat. This works best if you have a piece of cardboard and pushpins. Just pin the design right through the shirt onto the cardboard. I wanted the fuzzy effect, so I didn't really pin it down at all. Your image will be more crisp if you do though. Anyway! Go to work with spray paint. Make sure you have covered all of the edges of your stencil so as not to get any stray marks on your shirt. Again, I was not very careful about this. The spray paint won't stay forever, but it lasts through a year or so's worth of washes, and it works. You could always use t-shirt paint and a roller to accomplish this step. Whatever floats your boat. I'm an instant gratification kind of gal, so the spray paint gets my engine going.
4. Trim the shirt. cut out the neck, off the sleeves. Heck, make it a crop-top if you so desire. I probably woudn't have cut the sleeves, but they were kind of funny and tight at the wrists. I'm not knocking my shirt though, those fussy sleeves are probs the reason the shirt was on clearance for a $1 anyway. Put on your shirt. Love it.
Here's my finished product:
Also, notice the headband? I'm a freak for bows right now. Yes, the kind you wore when you were a kid. Somehow, they are a lot easier for me to keep on my head now then when I was a kid. I sprayed a ribbon of the appropriate size and length with the gold spray I used on the shirt. It was a little stiff, but it did the trick: accessorized my hair to match my shirt. Job well done, ribbon bow.
