You should proceed to the [Finance > Receivable Accounts] screen to find the transactions pending confirmation.

Once that is done, follow the steps below:

1. Find the desired transaction in the screen;
2. Click on [details];
3. Check in the [Account] field from which account will come the payment amount for the transaction;
4. At the bottom of the screen, click on the [Received] little box;
5. Fill in the requested information in the screen;
6. Click on [Save]
   

The ideal is that at the end of each day, your bills in the panel look like the example below: