Find some letters... I used these...
They were at Michael's in the scrapbooking section. I didn't shop around, so I'm sure you can find them cheaper elsewhere.
Find a piece of cardboard large enough for you message. Lay out the message.
Glue the letters to the cardboard, but remember to put the words backwards so they stamp forwards on your shirt. Also, you should probably use a ruler and measure it all out. I seem to do better eye balling it than with a ruler (I don't know why), so I didn't.
Cover the letters with fabric paint. I used black.
Test it out on a piece of paper first.
Then add a bit more paint (or blot off some) if need be.
Place the stamp evenly and firmly on to the shirt. Apply pressure, rub hand over the back of the cardboard without moving the cardboard... and then carefully lift the stamp off the shirt.
I added a touch of pink fabric paint to the heart just to add definition.
In future, I wouldn't use plastic letters because they don't absorb enough paint to stamp as dark as I would have liked. Again though, I didn't shop around... I just saw they were the right size for what I wanted to do.
I'm actually really pleased with how it turned out though, and will definitely try it again soon.