To use a home breathalyzer, insert a new mouthpiece into the breathalyzer, inhale deeply, and blow evenly into the mouthpiece for five seconds. Operation instructions vary depending on the breathalyzer, but they typically follow this process.
Turn the breathalyzer on after inserting the mouthpiece. Some breathalyzers have a countdown that lets you know when to exhale into the mouthpiece. The breathalyzer may beep after you exhale into it enough. After a couple seconds, it displays your blood alcohol content.
For the most accurate results, use the breathalyzer 30 to 90 minutes after consuming any food or alcohol. Food and alcohol contaminate the breath sample. Alcohol also takes 30 to 90 minutes to enter the blood fully, so readings taken sooner than this may be lower than the actual blood alcohol content.