When you are planning on installing any kind of corrugated roofing sheets, you need to put a vapor barrier down before hand. One of the cheapest and most effective kinds of vapor barriers for a roof top is tar paper. Tar paper is paper or other materials such as fleece combined with tar to make it water proof. This paper is somewhat delicate but when installed properly it keeps the water out very well.

There are certain techniques that must be used when installing this paper. If it is not installed properly, the roof will leak and not be waterproof enough to keep the roof from rotting away over time.
There are two different ways to fasten down tar paper. It can be attached with staples or button tabs. Button tabs are the better way to hook down tar paper. Button tabs are also more difficult and more expensive. It is all a matter of personal preference and the building codes of the area. Some require the tabs while others say staples are fine.

The procedure to lay down tar paper is not too difficult once you understand it. You start at the bottom of the roof and work up toward the peak. Start the tar paper on one side of the roof and put one tab or a few staples in it. Then roll the paper along the edge of the roof making sure there are no wrinkles and that you are stay consistent. At the end of the run cut off the tar paper with a utility knife. Set the roll off to the side and go back over the run to fill in all the gaps with staples or tabs. There needs to be special attention given to the lower edge where wind can rip up the paper.

After you get the run fastened down, you start the next run making sure it overlaps the previous by at least 6 inches. You continue this process until you are up to the peak. If the last run is long enough to come over the top you fasten the bottom and fold it back until you go up the other side. When you are done you will either have a piece from the first side long enough to cover the bare roof on the other or you will have pieces on both sides that need to be covered. If that is the case, you simply have to run one additional run splitting the peak of the roof. That will cover both sides and create a waterproof barrier.
This is the best method for laying down tar paper and will create a waterproof seal that will keep your roof in good shape. Just make sure to put the layer of roofing sheets down over top of the tar paper. The roof sheets will keep it from ripping off in the event of a very strong wind storm.