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.
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.