Select a shallow plastic container with drainage holes, such as a nursery flat or prepackaged-salad box (either clear or opaque plastic boxes work). Fill the container with 1½ to 2 inches of moistened organic potting mix. Cut a piece of cardboard to fit inside the container. Gently press the cardboard onto the soil to create a flat, even surface.
Related: Growing Salad Greens
Mesclun seed mixes are a good choice for microgreens: Varieties like Paris Market Mix and Asian Baby Leaf Mix from Renee's Garden contain a blend of greens selected because they tend to sprout at the same time. Single-crop microgreens also work. Scatter the seeds over the soil surface, then place the cardboard over the seeds and lightly press them into the soil.
Using a coarse wire-mesh sieve, sift a very fine (⅛-inch) layer of dry potting mix over the seeds. Set the container onto a drip tray and then carefully water the seeds in with a gentle shower from a watering can. Place the tray on a sunny, south-facing windowsill or under grow lights. Expect the seeds to germinate in 3 to 7 days.
Keep the soil consistently moist. Prevent soggy soil by pouring off any excess water that collects in the drip tray after watering. To harvest, snip the microgreens right above the soil line when their first true leaves unfurl (about 7 to 14 days after germination, depending on the green). The microgreens are nutrient-dense and make a healthy addition to salads and sandwiches.