Wait until it is time to water your plants again. Then mix the solution as per instruction, and water the plants as you normally would.
Mix one part 3% hydrogen peroxide with four parts water. Allow the top layer of your soil to dry, and then water your plants with this solution as you normally would. The soil will fizz for a few minutes after application; this is normal. The fungus gnat larvae will die on contact with the hydrogen peroxide. After a few minutes the fizzing stops and the peroxide breaks down into harmless oxygen and water molecules. Repeat as needed.
This really worked for me.
