How do we pay estimated taxes for income made only part of the year?

My husband recently started making independent contractor income that does not have tax withheld at the very end of May. He's only gotten 3 independent contractor paychecks so far this year and at this time we really don't have a solid idea as to what his income for the next 6 months will be.

 

How do we figure out what we need to pay in estimated tax payments for the 7.5 months of independent contractor income so we don't incur penalties?