Eventually, you will need to pay your bills. To do so, use the Process Payment application found in Business -> Vendors -> Process Payment.
The Process Payment application consists of:
Payment Info - Vendor - the vendor you wish to pay
Payment Info - Date - the date you wish to make the payment, normally the current date.
Payment Info - Amount - the amount of money to transfer in this payment.
Payment Info - Num - the check number.
Payment Info - Memo - any comments about this payment.
Post To - the A/Payable account to post this transaction.
Transfer Account - the account where the money will come from (a checking account for example) to make this payment.
Below is the GnuCash Accounts Receivable payment window.
Process Payment Window