*some white paint
*paint brushes some big, some small
*painters tape if you like
*print off a police telephone sign
*tape or tacks
*a step ladder
I'm so sorry the pictures are all wonky I took them in my phone and didn't take them right... Just turn your monitor on its side or something I guess
Step 1: PAINT IT BLUE
Step 2: Add Windows
Step 3: Hard Part
Now the top part is a little tricky. And I recommend you sketch it in pencil on the wall first. Reference your favorite tardis picture and try to to copy it. For those with less time than that it should be alright to just do about 2 inches of blue out from the end of the trim and then a stroke from the edge of the trim to the other edge and make a trapezoid on top of that and put a light. I'm positive that sounded confusing but if you look at the pic it might make sense. You can put lines in the light if you like at this stage or full it in first and then paint the lines.
Step 4: Here's Your Sign
Step 5: All Done!
UPDATE: I painted a police box sign on. It's not hard. You just paint a strip of black about 3 inches wide starting on the door frame and trim. then you carefully paint in the words. that way you can open it and it'll still look pretty good.