I found these insstructions at http://www.google.com/search?ie=UTF-8&oe=UTF-8&rls=com.google:en-US:official_i&gfns=1&sourceid=navclient&q=compostin+leaves+in+the+plastic+bags
EASY COMPOSTING

There are a number of recipes for plastic bag composting on the Internet. This article is a compilation of many. It includes informative articles from Texas A&M, University of Minnesota, and Nancy Szerlag. COMPOST is Mother Nature's way of recycling plant material into a substance that is beneficial to our soil. Compost improves soil structure by helping to break up heavy clay soils making them more granular, and increasing the water holding capacity of sandy soils. In addition, composting can, but not always, contain a good range of plant nutrients.

THE BAG

Containers for making compost vary from a simple chicken wire cylinder to a fancy rotating drum. But the simplest of all is a black plastic garbage bag. Plastic bag composting is perhaps the simplest of all composting methods requiring no structure other than a black plastic garbage bag. The bags should be 30 to 40 gallon in size and at least 3 ml. in thickness. This size bag should hold approximately 3 bushels of organic materials.

JUST ADD LEAVES

And the simplest and most abundant material to compost is leaves. A similar plastic bag (you can turn a multipurpose compost or bark chip bag inside-out) can be used to produce leaf-mold, but it needs to be perforated all over with a garden fork to admit air. You can either rake up the leaves, or collect them in the bagger of a lawn mower. Fill the plastic bag with leaf material, add one shovel full of soil - this supplies the needed microorganisms that ultimately degrade the leaves. A splash of water is needed to moisten the dry leaves and a handful of high nitrogen fertilizer to feed the microorganisms. If you mowed up the leaves, there is no need to add the fertilizer since the accompanying grass clippings will provide sufficient nitrogen. Give the bag a shake and place it in the shade. Then periodically flip the bag over to redistribute the contents and incorporate a bit of air. This method is a good alternative for those of us unable to turn a standard compost pile regularly, or have limited space in our backyard garden.

BETTER

Fill with a suitable mix of material, add a tablespoonful of balanced fertilizer (Grow more or Fish Blood and Bone), 1 liter of water and a cupful of lime, to counteract the extra acidity of the anaerobic process. Fill in situ for ease of handling. Close tightly to exclude air and leave for about six months to a year. No attention is required and the bag can be concealed behind shrubs.

BEST

For best decomposition, try to place a combination of both brown and green organic materials in the bag. To each bag add 1 cup of ammonium nitrate and about 1/4 cup of hydrated lime (in this case lime is used to counteract the acidity that will occur under anaerobic conditions within the bag). Then add about a quart of water, close the bag tightly and set it aside for six months to a year. You should not have to turn the mixture or add water after closing the bag. If possible, set bags in a sunny spot where they can absorb heat. In the winter, move bags to a heated garage or basement.

PATIENCE, PATIENCE

Plastic bag composting is convenient, but, like pit composting, the process occurs mainly under anaerobic conditions and much slower than composting in well-ventilated structures.