This is the simple way of making a hard thing.
Step 1: Planning and Materials.
Planning is an easy step. For this you will need some cardboard, a pair of scissors, a pencil, a tape measure, a bit of tape, a stick and a protractor, and then you get a model for the shape you want. A good finishing shape will be written in the next step so you just have to shrink the length, to the same percent. Use paper instead of cardboard. If you don't have a bit of cardboard that is big enough then go to a store which sells enormous objects, eg. a flat-screen TV, and ask if you can have a big cardboard box and chances are they'd say yes and be thankful that you got it off their hands. Then the planning step is complete.
Step 2: The Outline and Tabs.
On the box draw a 31 cm by 10 cm box. On both sides put a 9 cm line at an 140 angle. On both of those do another line at the same angle and that is the same length. On the next one do an 130 angle and then as long as 9 cm line. Then do another 130 angle for a 31 cm line. Now all you have to do is draw a line to the bottom bit of the starting point. Now you have most of the outline done so now you can draw the tabs. The sides are easy. On the bottom do rectangles that are 10 cm wide (or long on the 9 cm area) and all angles at 90. Then do triangles between all 'walls'. Just 3 more steps to go.
Step 3: Cutting and Putting Together the Project.
Cut around the outline, and go around the tabs and walls when doing so, as the tabs keep the coffin together and the walls, well are walls and they can conceal you. Then on the tabs, cut on the right side of them so that they don't get tangled in each other and then tape the tabs to the next wall together with a strong, clear tape. Then cut the lid in half and tape the bottom part down. The tabs really help so don't cut them off, whatever you do. P.S. 2 more steps.
Step 4: Foot-holes and Dressing.
Foot-holes are easy. Just cut a big oval in the bottom and there, done. To dress, wear a smart white shirt and a red tie. Wear a blazer over it. Black trousers help to hide your legs and so do black shoes. A better thing is to have chameleon shoes and trousers but I don't think they've been invented yet. If you don't have black shoes or trousers wear ones which look wooden, or wood hangs down. A good idea is to have a pillow or something soft and you will learn that in the next step. Wait, that's the last step.
Step 5: Acting and Make-up
Acting is pretty much the simplest part. You could stand up but it makes a better impression on people if your coffin is lying on the ground, or better, on the steps. Now do you understand the pillow on step 4??? Now for make-up. Get some white cream and make your face pale, and then get in the coffin. Now with this last step in mind, scare somebody!