Soft, nutrient-rich grass clippings may seem like an ideal source of organic matter for the garden, but unless they're mixed with something fibrous, they soon mat into slimy, useless clumps. Instead, use them to turn inexpensive, low-nitrogen materials such as sawdust into useful compost.
Gather grass clippings from your lawn, or someone else's, that have not been treated with weedkillers. Using a grass catcher on your lawn mower is an ideal way to do this.
Mix the fresh clippings with two to three times that amount of dry leaves, sawdust or straw in a compost bin. Within a day or so the mixture will start to heat up, indicating decay--and the creation of compost--actively taking place.
Stir the pile every day or two with a pitchfork. You can also take out the material, remix it, then return it to the bin. This aerates the pile and speeds up the process of decay.