how to pay for unpaid OT?

The answer is a little complicated, but generally speaking, the amount you were paid is spread over the number of hours you worked (regular and OT) and divided to get the average - and that is your hourly rate, what is then owed is the 1/2 that was not paid for every hour of OT worked.  This answer may change if you have a contract or something in writting as to your hourly rate.  Again, it is more complicated than that and you should contact an attorney as soon as possible becuase there are various time limitation that may pertain to your case under the different Federal snd State laws.