You can purchase clover seeds at garden stores, at most home and hardware stores, and online. You'll need roughly 2 ounces (57 g) of clover seeds per 1,000 square feet (93 square meters) of lawn.  The most popular types of clovers for lawns are Dutch White (a perennial that grows to 8 inches, or 20 centimeters) and Microclover (a durable clover that has small leaves and short stems).  Inoculated clover seeds of any variety are also popular, because it means you don't have to inoculate them yourself. Inoculation is the process of coating the seeds with a nitrogen-fixing bacterium that allows the clover to produce its own nitrogen. Be sure to store these seeds in a cool, dark place. The best time to plant clover is in spring or summer. To plant in spring, make sure the last frost has occurred, and that nighttime temperatures are staying above 40° F (4° C). The best time to seed clover is between mid-March and mid-August.  In warmer areas, you can plant clover in September and October, but plant it at least six weeks before the first frost. In the warmest areas where winters are mild and there's rarely snow or freezing temperatures in winter, you can plant clover all year round. Depending on the size of your lawn, you may have conditions that vary greatly from one area to another, so some areas may not be ideal for clover. Clover is very hardy and will even grow in partial shade, but it will do best when it gets between four and six hours of sunlight per day. Where possible, prune and trim trees, hedges, and brush to ensure the clover gets enough light.
Buy clover seed. Plant according to the season and weather. Choose a sunny location.