This is a very important skill, which needs to be mastered before most other projects can be made to turn out nice.
You now have a 4"(minus a bit) by X" sheet. If that's all you need, you're done! If not, just repeat steps 5 - 7 until it's as big as you need it.
It is advisable that you trim the edges so they look cleaner, unless you can magicly rip every piece of duct tape exactly the same length.
A different way to lay sheets of duct tape which end up being much stronger than this method is to alternate the direction of the strips on the front and back. The downside to this is that you need to lay all the sticky side up tape first before you can start covering any of it, so obviously at some point you will need a larger sheet of duct tape than this method and space/time allows. Start by laying strips of duct tape left to right, overlapping the strips. (although as little as posssible is needed.) Once you have all the strips layed facing up, cover them with strips running top to bottom, at a 90 degree angle to the tape you laid already. Do this until all the sticky sides are covered and you now have an almost indestructable sheet of duct tape!