Begin by removing the spark plug wire from the spark plug (this is generally located towards the rear of the lawnmower, facing the handles). This will stop the engine from accidentally starting whilst you are working on it. You will need to remove the spark plug from its opening (a socket wrench is usually the best tool to do this with). If your lawnmower engine has been flooded, the tip of the spark plug (that was inside the lawnmower) will appear wet and have a strong odor of fuel.
Spray this tip with some alcohol-based engine starting fluid; this will dry out the fuel-soaked spark plug. If the tip is black, dirt is present in your lawnmower engine. Spray it again and give it a good scrub with an old toothbrush to remove most of the grime. Allow the spark plug to dry completely. As the starting fluid will evaporate rather quickly, this shouldn't take long.
Make sure that the throttle is completely in the 'off' position, or the slowest setting on your model of lawnmower. By giving the start cord a few pulls, you will help to dry out the inside of the combustion chamber. Never spray the engine starting fluid directly inside the spark plug opening, as this can cause serious damage to your lawnmower.
Insert the spark plug back into the lawnmower's engine and tighten it. Make sure that you follow the proper tightening instructions, as over tightening can cause the spark plug to break. Make sure that the throttle is in the 'start' position and give the start cord a pull. The lawnmower, if correctly fixed, should start.
If your spark plug does not appear to be flooded or it still won't start after you have attempted to fix the issue, your lawnmower may be experiencing a more serious problem and should be taken to a professional service centre to be looked over. If you do, however, succeed in fixing your flooded lawnmower, you may find your friends and neighbours coming to help you fix theirs.