Firstly open a Patient Chart by either searching for a patient in the search box at the top right side of the screen or from the Patient List in the patient menu at the top left side of the screen.
Now click on Billing situated second in position at the left side menu.
Now click on +POS for billing additional payments.
Now fill all the required details and click on Pay.
Here you can charge the billing amount if full or in less than that.
If you have charged less than the billing amount you can again charge the amount by clicking on the +Payment
For a more detailed article on billing