When sowing petunias in a seed tray, it is best to do it indoors as the seeds are very fine indeed, and any wind will blow them everywhere. Make sure the compost is damp, and when you open the seed packet, tap the seeds carefully in the seed packet to the end of it, then tap the seed packet very gently with your finger, while moving your other hand to make sure they are evenly distributed. Do not cover them with more compost or water with a big watering can. Use one of the little hand-held mist sprayers that people use for houseplants.