If your store-bought lemongrass has no roots
Cut off the tops of the leaving only about six inches of the lower portion of the stalks. Soak the trimmed stalks in a jar of water so that the lower half is submerged. Keep the jar where it gets plenty of sunlight and wait for a few days for roots to grow.
If the lemongrass you bought has roots

Simply cut off the top portion of the stalks and plant directly on soil.
Re-planting lemongrass
Plant directly on well-draining soil in a part of the garden that gets plenty of sunshine. Water once a day.
If you’re have several stalks, plant them at least six inches apart. Lemongrass is a grass. The replanted lemongrass will grow the way grass grows. Horizontally and vertically. The dark green portion of the stalks will regrow and new stalks will sprout around the original plant.