I always wanted to make a TARDIS (not the real one yet) of my own. But I always try to find the laziest way to do things. Thus after sleeping on it for some time, with some scrap materials I tried. This is my first instructable and I will be very happy if it helps.
So without much talking, off we go!!!

Errrr, I meant "ALLONS-Y!"

(English is not my native language, so I will try my best not to make you mad with many mistakes but apologies in advance if I do.)

Step 1: Supplies

An unused cardboard box. I took one which was sitting in my room for many years containing my Pokemon cards. You can use milk cartoons on anything whose shape matches the TARDIS.
A scissors
Glue. Just craft glue. I prefer liquid craft glues over glue sticks because I think they hold better.
Sketch-pens of the shades of blue. If you have seen the TARDIS, explore with the color blue.
Black Marker.
Ruler, pencil.
Cap (preferably white) of a toothpaste tube or of any tube big enough to be the top lamp of the blue box and.
Some Tape.
Art paper or craft paper of light blue shade.
Once you have read this fully, feel free to add your own parts to the TARDIS.

Step 2: The Roof

The roof of TARDIS is not flat.
In some it has steps which moves up like a tomb and in other there is a sloping nature (Pic 1).

I hope I won't get you lost through my writing.
Pic 2-4 : Draw a cross on the top of the box drawing the diagonals and then add a little square at the center as shown.
Pic 5 : Cut along the lines starting from the four corners but only up to the square. Don't cut in side the little square.
Pic 6-7 : fold along the cuts for easier access to the cut lines.
Pic 8 : Nip a bit around the lines and make the slits wider.
After nipping it will look like Pic 9 if you open the lid of the box.
Pic 10-11 : Now press the sides of each cut line close to each other and tape along it.
This will create a slanting nature to the lid and it will look like Pic 12-13.
Pic 14-15 : The box after closing the modified lid.

Step 3: Cover Up

Once the roof is done, its time to Cover the whole old shaggy box to give it a new life.
Cut the Blue paper in right sizes and cover whole box.
Sorry I didn't take enough pictures of this step.

Step 4: Its Coloring Time!!!

Follow any particular model of TARDIS which you like and draw it with ruler and pencil.
Once it is done, start coloring with the sketch pens.
When blue parts are done, give it some shades and parts with the black marker.

I know we can use paper of the particular shade instead of coloring it or even we can print the pattern ready-made instead of all this work but I wanted to make it a little raggidy - type. I liked how Tenth doctor had a bit old looking TARDIS. And the sketching gave it somewhat 'wooden look' [ I know its not much :'( ].

Step 5: Finish It Your Way

Once it s colored, it is up to your satisfaction how much detail you want on it!

Stick the toothpaste cap on the top of it and voila! You have a TARDIS!
And you can also store anything you want because it is still a functional box.

[ Try placing an elephant inside because *snort* its bigger on the inside *snort* Muahhahahaha sorry my bad ;) ]