Soak the star fruit seed in a bowl full of cool water for an hour before sowing it. Prepare a growing container for the seed while it soaks. Fill a small, 4-inch-deep nursery planter with commercially prepared potting soil, or create your own growing mixture with 2 parts coir, 1 part loam and 1 part perlite. Leave a 1/4-inch gap between the soil and the lip of the nursery planter.
Place the star fruit seed on the surface of the soil in the center of the pot. Cover the seed with a 1/4-inch-thick layer of soil. Press the soil firmly onto the seed to anchor it. Spread a very thin layer of fine sand across the surface of the soil in the pot to help regulate moisture loss from the soil. Drizzle water into the planter to settle the sand and moisten the top 1/2 inch of soil.
Place the potted star fruit seed onto a propagation warming mat near a very large, bright window. Set the temperature on the warming mat to 75 degrees F. Drape a portion of plastic wrap across the top of the pot to mimic a greenhouse environment. Keep the soil in the pot moderately moist at all times while the star fruit seed germinates. Remove the plastic wrap and drizzle water onto the soil whenever it feels dried out on the surface.
Watch for germination seven to 14 days after sowing the star fruit seed. Lower the temperature on the propagation warming mat to 68 degrees F once the seed germinates, and decrease watering slightly. Allow the top 1/4-inch of soil to dry out between waterings. Keep the star fruit seedling in its original growing container until it grows to 4 inches in height, then transplant it into a permanent pot or bed.