Use a sterile, unopened petri dish for culturing (because we only want what we have inoculated (deliberate introduce specific microbes), otherwise the result will be inaccurate and invalid.)
Prepare another sterile, unopened petri dish as control (for comparision for the experiment)
Seal the lid shut with 2 tapes on both side to keep the lid closed but not exluding the air. (1. NEVER open the petri dish again as millions of microbes will be released 2. We do not fully seal the petri dish to provide oxygen for aerobic microbes while suppress the growth of harmful anaerobic respiration.)
Incubate upside down (prevent condensation dropping on the colonies and mixes them up).
Incubate at cool temperature (25 °C) (reduce chances that pathogens are growing & reproducing quickly under optimum temperature of 37 °C )
Observe the shape, color, and size of the colonies with the naked eye.
Dispose agar plates appropriately. They should sealed and incinerated (burned completely).