How do you charge overtime when working on several different jobs in a week?

I found this and it should be what you are looking for. Just make sure you do it each week.

Generally, the federal Fair Labor Standards Act (“FLSA”) requires that where work is performed at two or more rates, overtime must be compensated at a blended rate. “Blended rate” means a rate of not less than one-and-a-half times the weighted average of all non-overtime rates used during that workweek. For example, let’s assume an employee cut/styled hair for 20 hours, washed hair for 20 hours, and scheduled appointments for 10 hours in a workweek. The blended rate would be determined by the following equation: 20 hours X $20 + 20 hours X $15 +10 hours X $10 = $800; $800/50 total hours = $16 per hour. Therefore, $16 per hour is the blended rate. The premium pay for overtime is then half of $16, $8, multiplied by the 10 overtime hours for a total of $80. Total compensation would be $880 for this week.