This article is from the Care of Orphaned Kittens FAQ, posted to rec.pets.cats newsgroup. Maintained by Sharon Talbert with numerous contributions by others.
By four weeks old or a bit sooner, your kitten can be introduced to solid food. Start with a slightly warmed moosh of formula mixed with strained meat babyfood (chicken or turkey) and formula, offered on a saucer or small plate. (There is a transitional cereal offered by Just Born you can mix into the mess as well.) Be sure not to overheat the stuff in the microwave -- only a few seconds is all it needs, and be sure to mix it thoroughly with your finger so that you get all the hot spots. You may have to put a bit of food on the kitten's nose or in its mouth to get it going, using your finger or a plastic spoon. Within the week, add a good-quality kitten chow (I prefer Iams), softened in warm water, while phasing out the formula, both by nurser or in the solid food (moisten with water, as necessary). By the time the kitten is six weeks old, it should be scarfing down straight kitten chow and drinking water on its own. Wean gently and gradually though; you don't want a thumb-sucker on your hands.