what you'll need:
-freezer paper
-parchment paper
-scissors/x-acto knife
-t-shirt
-iron
-paint/paint brush
first, tear a piece of freezer paper roughly the size of your shirt or design. you'll need a piece of parchment paper about the same size as well. i just sketched my idea onto the matte side of the freezer paper but it's probably better to draw it on something else and then trace it onto the freezer paper.
next cut out your design, this can be pretty tedious depending on your design. sadly the sun went down while i was cutting so all my good lighting went out the window...ah working in the dark because i'm too lazy to turn the light on... if you have a lot of fine lines in your design i'd recommend making them a little bit thicker than you think you need to.
plug in your iron and set to medium/high heat.
now, place the parchment paper inside your shirt directly underneath where you'll be placing your stencil. line your stencil up on top of your shirt, shiny side down. using a press/lift/press motion (not a sliding motion) iron your stencil to your shirt. make sure you get a good seal otherwise your paint will leak. if you have any separate pieces or free floating pieces iron them last.
now for the paint, i used a speedball screen printing paint, but you could probably get away with acrylic. i mean any time i get acrylic paint on my clothes it never comes out... i used more a dabbing motion rather than actual strokes mostly because i didn't want part of the stencil to catch on the brush. now wait for the paint to dry...
...
double check you didn't miss any spots with the paint, and once it's dry you can peel off your stencil.
if you need to heat set your paint, just use the same piece of parchment paper, place it on top of your design and iron for 3-5 minutes. keep the iron moving to avoid scorch marks. flip your shirt inside out and repeat on the back of your design.
and your shirt is ready to wear!!