Apply a check payment to a/p credit

How do I apply a check payment to a credit I have in accounts payable?

You can do it through AP by going to the vendor and creating an entry for the amount of the check but instead of using your expense GL code you would use your cash account. Then write a check against the credit transaction and the new entry  you made to zero out. Or you can wait until you have more bills and just include both transactions on that check.