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