Planting the Seeds
Fill a plastic planting tray with commercial potting soil. Scatter petunia seeds lightly on top of the soil. Go easy, because petunia seeds are very tiny, and if the seedlings are too dense, they won't receive adequate light and air circulation. Press the seeds gently into the top of the soil, and spray the soil very lightly with a mister.
Cover the tray with clear plastic wrap and put a Popsicle stick in each corner to prevent the plastic from resting on the soil. Put the tray where it will be warm but not exposed to direct sunlight. The temperature should be at least 70 degrees, but not more than 85 degrees.
You probably won't need to water the seeds again until they sprout, but keep an eye on them. If the soil appears dry, remove the plastic, mist the soil very lightly and replace the plastic.
From Seed to Seedlings
Remove the clear plastic after the petunia seedlings emerge from the soil in a week to ten days. Put the tray in a place that is bright and slightly cooler, about 60 degrees. Florescent lights are ideal for growing petunias, and any fluorescent tube will do. Put the light over the tray, leaving about 6 inches in-between.
Leave the petunia seedlings under the light until they grow three leaves, and then transplant them to small individual pots, with two or three seedlings to each pot. Water them every other week with a liquid fertilizer mixture.
Planting the Petunias Outdoors
Wait until the weather turns sunny and warm and until all danger of frost has passed. Harden the petunias first by putting the pots in a shady spot outside on warm days, but bring them in at night. After you've done this for a few days, start putting the petunias in full sun for a few more days.
If you have a slug problem in your garden, deal with it before you move the petunias outside, because hungry slugs can make short work of young petunias.