So, I have decided to take the plunge and try it myself, starting small and keeping it simple.
What you will need: A plain T-shirt, A4 Card-board (I have used an old Cereal box, Honey Nut optional:P) and a Fabric Pen (Black).
2. Because I wasn't confident at drawing the Cross free hand, I drew a rough one on the cardboard as a guide (See below).
3. Then I placed the card board in the middle of the T-shirt so I was able to see the cross through the material.
4. With the fabric pen I roughly drew the cross onto the T-shirt. While doing this I found that the pen wouldn't stay in a straight line, but this actually looks a lot better so i just went with it.
5. Then repeat to finalise your cross.
And there you have it, an individual T-shirt. (For care details see the Fabric pen as these may vary.)
(Sorry about the dodgy pose it was late and I was tired!)