You will need
*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
paint the door and surrounding trim blue. let dry
Step 2: Add Windows
Paint two white rectangles about 3 inches from the top of the door and about four inches apart. Mine are 1'7" by 9" but just so long as they look proportional. don't worry about dribbling on the blue you can fix it later.
Step 3: Hard Part
Here you'll add the panes and the topper. Painters tape would be useful here or you lines might be slightly wonky. For the windows remember it's two panes down and one across.
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
Print off your sign and either track it or tape it up. I used electric tape and just painted over the black but any way works really.
Step 5: All Done!
Post pics of your new door and comments that let me know how I did. :)
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.