How do i write a statement to declare past overtime hours worked to get backpayed because of the new salary exempt law changing the minimum?

1) Your rights: if you were not exempt from overtime--such as for being paid too low a salary under the new rules--you are entitled to overtime for all hours worked past 40. For a salaried employee, determining overtime can be somewhat complex and, depending on certain assumptions made, answers may vary, but to give an example and oversimplify the math: let's pretend there are only 50 weeks in a year; that means that your weekly salary is $600. If 40 hours is your company's normal workweek, your equivalent hourly wage is $15/hour. Say you work 50 hours in a week. For 10 of those hours, you get an overtime premium equal to 50% of your hourly equivalent wage, or 50% x $15.00 x 10 hours = an extra $75 that week.
2) There is no set form for such a statement: just try to be as accurate, clear, and concise as possible. 
For example: "I generally get to work to 9am and work until 6pm; while I sometimes may leave a few minute earlier, I also sometimes leave later, so working 9 - 6 is a reasonable average. So any week that I did not take vacation or sick leave, or in which there was no holiday, I would have worked 9 hours per day for 5 days, or 45 hours. Therefore, I work 5 hours overtime per week on average."
Or: "Most weeks I do not have overtime. But the following weeks, when I was working on the [insert project name] project, I generally worked until 8pm because we were under a tight deadline. Therefore, those weeks, I worked 55 hours each work, or 15 hours overtime." 
Etc. Back up the statement with any documents (including emails) or details which support what you write, especially for weeks with more overtime.