More than likely, you've been unschooling your preschooler all along. What is unschooling? According to Unschooling.com, "Generally, unschoolers are concerned with learning or becoming educated, not with 'doing school.'" In other words, unschooling is basically just organic learning. It's learning that isn't necessarily intentional, though unschoolers facilitate their child's natural interests and desires by creating a learning environment for them. For pre-K, consider reciting the alphabet daily and focusing on one letter per day. Talk about the day's weather and the days of the week. Try asking, "If today is Wednesday, then yesterday was Tuesday and tomorrow will be...?" every day. For math and special relations, use Legos, blocks and other counting manipulatives. Starfall.com is a very popular and free website that is perfect for preschoolers.
Preschoolers tend to get focused on one particular topic; they love to learn or talk about that topic as much as they can to anyone who will listen. For this reason, unit studies are perfect for them. Unit studies are also great if your preschooler isn't the only one you're home schooling; you can teach multiple grades at the same time with unit studies. If you prefer, you can put together your own unit studies. Say your preschooler is interested in space and you decide to do a unit (which can last as short or as long as you and your child want) on space. Go to the library and check out many nonfiction books on space in the children's section and maybe a couple in the fiction section. See what videos and DVDs your library has to offer on the topic as well. Then, come home and look up some interactive websites that are all about space, such as NASA's Kids Club. Depending on your child's level of interest, you might create a diorama of the solar system, or you might have him fingerpaint a picture to depict the solar system.
If the idea of putting together your own unit studies seems intimidating, websites such as the one by Amanda Bennett offer many suggestions.
When most people think of home schooling, they think of sitting their child down at a desk (or the dining table) and opening up textbooks and workbooks and teaching with a whiteboard or chalkboard. If the thought of doing this makes you more comfortable, consider using a traditional curriculum. The Rhode Island Guild of Home Teachers has a list of some of the more popular publishers of traditional curricula.