To add a subscription payment:
In the subscription window, click on the green "+" button.
Set the date that the payment is scheduled to be paid. Typically this will be set for a future date.
Set the rate amount. In this instance, the rate amount will be $300.00.
Click Create, and the payment should be added to the subscription payment series.
On the right side of that line, click on the cogwheel, and select "Add Charge".
In the dialog box, click on the drop down menu, and select the charge that you'd like to add to the payment. When the appropriate charge is selected, click Create. In this example, I'm adding the "Boat" charge).
The charge should now be added to that payment. If the wrong charge was selected on accident, you can click on the trash can icon next to the incorrect charge to remove it from the payment schedule. You can only remove charges prior to payment. Once payment has been received for a specific subscription payment, you are no longer able to remove the charge).
Once all charges are added, the balance for those charges can be paid via the subscription payment cogwheel, and selecting "Pay Now".