Mix equal parts cactus soil and peat moss together in a large planter.
Fill the planter 3/4 full with the cactus soil and peat moss mixture.
Place the cactus seedling in the planter so that its roots rest on the soil. Then fill more of the cactus soil and peat moss mixture around the roots. You want the cactus' roots to be below the soil, but the seedling's green stem should be above the soil.
Put the planted cactus in a room that is consistently bright throughout the day, but has low humidity.
Water the saguaro seedling whenever the soil dries out completely to the touch. Depending on the conditions in your indoor location, this should be between 10 days and two weeks. Press your fingers into the soil to 4 inches. If you feel any moisture in those first 4 inches, wait a few more days to water until the first 4 inches are completely dry.
Fertilize the cactus just once in the spring with a well-balanced fertilizer.