I went to Jacksons in Northbridge (a block or so up Barracks Street from the museum) and gave them a laser printout of the design which they turned into a screenprinty thing. I attached this to a cardboard frome, flattened the shirt over a big book (with paper between!) and screenprinted on black paint using a little thingy the guy at Jacksons persuaded me to buy.
Once this was dried and ironed (per fabric paint instructions) I painted in a mix of "cool yellow" acrylic paint and this stuff that turns acrylic paint into fabric paint. This was a pain.
Once THIS dried I painted on extra black in any gaps/places I'd gone over in yellow. Then I let it all dry (smudging the speech bubble in my impatience) ironed it one last time and then gave it a bit of a wash and dry.
From K-mart: shirt, about $15.
From Jacksons (most of these had a 10% student discount I haven't included):
Check your design with a light behind it (my ironing board did this well) in case there are subtle gaps in colour.
Make sure the design is exactly where you want it (mines a bit to the side)
Don't stress over little blobs. They're inevitable and not really noticable.
Dark colours on white is much easier than vice-versa. Especially since the fabric paint is much cheaper that way.