Screen printing is awesome: it lets you print the same design multiple times and it gives you clean lines with fine detail and islands. But if you just need to make one of something - a decal on a costume, a silly t-shirt, whatever - screen printing can be a lot more than you need. Stencils are quick, but more limited in terms of detail and islands.

Enter freezer paper and spray paint!

This works only as a one-off, since you're making an iron-on mask out of the freezer paper, not a stencil, but you can get as much detail as you want to cut into it and islands totally work.

The spray paint works just fine as fabric paint, and cures super quick! Wear it the same day if you don't mind the fumes (not OSHA approved).

Step 1: Supplies and Concept

For supplies all you need is:

Freezer paper, shiny on one side matte on the other. Waxed paper isn't recomended, it's just too waxy.
X-acto knife or razor blade(s)
Cotton shirt. Other fabrics work as well, but cotton is nice because it will absorb the paint a bit and keep it from sitting on the surface and cracking. Coated and synthetic fabrics won't absorb paint, so they are a bit tricky. Test before you go full scale!

The color and absorbency of the fabric is pretty key to this process. Light fabrics work better than dark fabrics, because the the paint is sitting in the fabric - there's no coverage, it's much closer to a dye process than a paint or printing process that would leave a layer on the surface of the fabric.

Circa 2008 I wanted to make a T-shirt that I could wear to a staff party. My constraints were that it had to be wearable in two days and I had no money to spend on the project. I worked as a computer lab monitor for my college, and we had a large number of signs in the lab asking students not to eat in the lab, save often, printing costs 10c a sheet...you get the idea. I decided to make a shirt that featured a sign that had dramatic consequences if disobeyed. Simple is better if you're on a tight timeline, so I decided to adapt an existing trip hazard sign.

Step 2: Make the Mask

Not a stencil! The sweet part of this is that you can have islands (free floating regions not connected to the stencil). In practical terms, it lets the inner triangle float inside the outer and look awesome.

For this mask I decided how large I wanted the center image to be, printed it out, and then used that to decide how large the text would end up. I ended up making the text fairly large, as anything below an inch high is pretty time consuming to cut and hard to see.

As part of the quick and dirty, I did all the layout in the flesh, and just used the computer to get images and print text. My hand lettering is terrible, so I used a stencil font to match the industrial look of the sign.

Once you have all the elements printed out, arrange them in their final positions on a sheet of freezer paper, with the shiny side down. This is how the final mask will look, so make sure you have everything where you want it. Tape everything in place, then start cutting! Single edge razor blades are my preference, because nothing is worse than a dull blade, and single edge razors are super cheap.

The advantage in laying everything out this way, for me at least, is that it saves having to build the image in Photoshop, printing the image out as several pages, then trimming the edges carefully and aligning all the joins. I just rough everything together on the freezer paper and it's ready to cut.

Step 3: The Fun Part

Once you have everything cut out, pull off any remaining paper and tape from the freezer paper. You are going to hit it with a hot iron, and tape will melt and ruin all that hard work your iron.

Iron out the shirt. If you don't have an ironing board some towels and a piece of plywood works fine. Next, position the freezer paper so that the mask is where you want it to finally end up. As you can see above, my mask is up in the middle of the shirt - make sure you position any islands also.

Start ironing the freezer paper down on medium to high with no steam. Working out from the center will reduce the number of wrinkles. and help you avoid missing areas. The entire sheet does not have to be ironed down, but get the edges and area around the mask stuck well, or your lines will be messy.

Once you've ironed on the mask, take it outside and shake up your paint. Slip a sheet of cardboard inside the shirt to keep the paint from bleeding through, then start painting. Light coats are best, if you lay it on too thick the paint will wick under the mask and ruin your day. (Note that spray paint does have a habit of getting everywhere, use newsprint to make sure anything you don't want paint on is covered.)

Once the paint is dry to the touch, peel the freezer paper off and you're ready to go! The freezer paper is one time use only, so just rip it off. You can wear the shirt as soon as you want, but let it cure for a few days before you try washing it. I've machine washed my shirt for the last 4+ years, and it's held up surprisingly well. Your mileage may vary depending on your fabric, but for cotton, machine washing is just fine.

Step 4: Notes and Variations

Paint

Spray paint isn't good to breathe.
Plastic-compatible spray paint might work better on synthetics?
This method also works just fine with normal fabric paint and a brush - or a sponge! ashleyjlong pointed out in the comments that this gives you better control, and has no over-spray.