Make sure you have a recent backup of your system before you begin this process.
In Entry, Account Entry create an Accounts Payable liability account if one does not exist. Check the Accounts Payable check box.
In Options, Define Year and Month. Enter a default Accounts Payable account.
Click the Change Accounting Method button.
The system will ask you to confirm, click Yes.
Restart CMS for all changes to take effect.
From that point on, all bills must be entered to the Accounts Payable account instead of the bank account. Even for hand-written checks.