Mesclun is a mixture of various quick-growing greens that are harvested when they're young and tender. Seed blends usually include a variety of mild and spicy greens, often including both red and green varieties for visual appeal. Seed blends with lots of arugula and mustard or mizuna will have a spicy kick, while those that favor leaf lettuce and chard will have a milder flavor.
Sow seeds directly into your garden or raised beds, broadcasting them evenly over the area. Try to space seeds about a half-inch apart. After sowing, cover seeds with about 1/4" of planting mix and pat the area gently to ensure good seed-to-soil contact. Water carefully with a fine spray so you don't dislodge or wash away the seeds. Keep the seed bed moist and seedlings will begin to sprout in a week or so. Be patient, as some of the seeds in the seed mix may take as long as two weeks to germinate.
When seedlings are 3" to 4" high, begin harvesting. Use scissors to cut the leaves about 1-1/2" above the soil line, taking care not to cut plants back too far. The plants will resprout and will be ready for another harvest in a few weeks. Expect to get three or four harvests from one planting. For a continuous supply of young greens, sow a new section every two weeks or so.