1. Dig a hole. 2. Put the seeds into the hole. 3. Fill the hole. 4. Wait until it grows.
EDIT BY DROPHYD167B:If you're talking about planting seeds in the video game, Ratchet & Clank Future: A Crack in Time, which, judging by the question's category, I guess you are. Then you pick a seed up with your wrench, find the patches of soil, there are three each time you can plant seeds, target them, and throw them in there with "R1" or the square button.