Set up your lab space. Take special care to set up your lab space away from air movers like heating and cooling vents or fans. Moving air can blow contaminants across your agar medium and ruin your experiment with false results.
Warm your nutrient agar to room temperature. Agar is commonly stored in the refrigerator, never the freezer, but it should be at room temperature before setting up your medium. Place the agar upside down to avoid condensation contamination and let it warm slowly.
Sterilize your loop. Place your loop in a fire to sterilize while holding the handle with a pot holder or heat resistant gloves. Let the loop turn bright red before removing it from the flame and allow it to cool for 10 seconds before collecting your specimen. Sterilize your loop between each collection.
Run your loop around your collection area. Tilt open the petri dish only enough for the loop to fit in underneath the lid to avoid contamination. Spread the loop across the surface of the agar, do not puncture the surface of the agar. Spread the loop across the agar in a pattern like an "X" or a zigzag. This allows you to see clearly if the growth is the result of your inoculation or from airborne contaminants.
Re-cover the petri dish immediately after you have finished spreading your collection material across the agar. Tape the lid closed on the petri dish and place it in a ziploc bag if you are worried about further contamination or smells.
Store you petri dish in a warm area, upside down, for two to three days. You should see growth after that period of time. If you want to take your experiments a step farther, you can use a gram dye kit to determine what kind of bacteria you have cultivated.
Dispose of agar dishes properly once your experiment is concluded. A school should have policies in place for disposing of potentially hazardous materials. For at home experiments, dip the petri dishes in a bucket of bleach and throw them away in a sealed trash bag.