Many people don't realize just how much air escapes through their attic and how far an insulation tent over the attic door would go in retaining warm air and reducing overall heating expenses. All you're really doing is providing a protective layer over the door, so it's not a terribly complicated procedure, and, based on how inexpensive it is to make one versus how much money it will save you on heating bills in the long run, it only makes sense.
Step 1 - Cut the Foam
Cut the foam to size, so it'll fit over the stairs. Measure the sides first. You need four pieces for each side, and they should be tall enough so the top piece can close completely. Take care not to make them too tall, however; the taller they are, the more open space you'll have from the attic door to the top of the cover, and the less effective a job it'll do. Once you have the measurements for each piece, use the utility knife to cut them out. Remember to be careful when using sharp tools.
Step 2 - Connect the Pieces
Once you have all the pieces cut out, the next step will be assembling them all together. The first two pieces are the trickiest, unless you have someone to hold them up for you. You need to connect a long side piece to one of the short side pieces using heat-resistant duct tape. It can be difficult getting the first two connected securely since you're balancing them both on their sides and working the tape, as well, but once you have the others, you can go back and readjust them.
Use the tape to connect the remaining side pieces, then use the caulking gun to apply a thin bead of caulk. It doesn't need to be thick; just enough to cover the corners is fine. After putting the sides together, lay the top piece on top of them. It will look like a rectangular box with no bottom at this point. Line the top up so there's minimal overlap and place the hinges on the side that you want the top flap to open at.
Use the screwdriver to install the hinges. When you're done, you can put a small amount of caulk on the screw holes if you want but it's not necessary. Once you have it all put together, set it aside and let the caulk dry.
Step 3 - Install the Cover
Place the cover in the attic door from the top. It should fit snugly. Apply caulk around all the edges to secure the tent and create a seal on all the edges and corners. The only step left is remembering to shut the cover door behind you.