Gather supplies. You shall need the following:
-A shirt. I got mine from wet seal because they always have good sales and I got this for about 3$.
-Fabric Paint-michaels and joannes
-A sponge-y brush-michaels
-Wax paper-wegmans
-tape
-scissors
-a pattern
-sharpie
-Exacto knife

sketch out your design, and arrange design/pattern on shirt.

(I know the TARDIS has 4 panels but I ran out of room)

Trace pattern onto wax paper with Sharpie. (tis evil I know)

Cut out the "negative space" of your pattern. It's kinda like carving a pumpkin. You know those designs that you use on the pumpkins? And how you cut out the black part? It's like that. So here, I cut out the letters and the TARDIS Shape. I did the TARDIS in 2 layers. I did the box itself in blue,let it dry, and then did the detailing of the doors and panels and such.

Place the wax paper on the shirt the way you want it and tape the edges of the wax paper down.

Here's the first part of the TARDIS.

By the way, here are the brushes I used. The main one I used is the one in the background...the circular one. The big one smack dab in the middle of the picture is what I used to go around the TARDIS.

Now carefully sponge on the paint.

Let dry for 10 min and peel off. Or do a 2nd coat. You can peel the wax paper off when it's wet If you want but then there's a greater chance of the excess paint dripping onto the other part of your shirt. which as you can see, I did. But I kinda wanted it like that because I wanted a "just spraypainted" kind of look.

Do this for the rest of the pattern.

Here's where I put on the 2nd layer of the TARDIS. I did it freehand with a regular paintbrush. But I wouldn't recommend doing that. Usually I make a 2nd pattern of the design and cut out where I want the 2nd color to be.

Here I used the huge sponge around the TARDIS, but if you aren't using this technique Then don't worry about it, and skip this step.

Let your shirt dry.

Now follow the instructions on the back of the fabric paint bottle. Mine said to put a piece of paper over where I used the paint, and to iron over it. I did, And now you are done. :)