When the client opens the bill, they can click the "PAY BILL" button at the top right.
In the "Make a Payment" window, your client should add their credit card information and address (must contain a valid State). The client can then modify the amount in the "Payment Amount" field to make a partial payment if desired.
For more information about Clio Payments, click HERE.
For more information about Clio's classic online payments, HERE.