Store credit is treated almost exactly like any other payment method and can be used just as easily.
Add a customer. Here is a link if you need more help.
Add Items to transaction
Click "Tender" at the bottom of the screen
Click the "Store Credit" Payment option
Click "Add Payment" Note: this will charge the full amount of the transaction to the store credit balance. If the balance doesn't have enough funds the transaction will error. Click here for how to use store credit for a partial payment and add an additional payment method.
Click "Complete"
The Nitty Gritty
A customer must be assigned to the ticket in order for a store credit transaction to work. If you forget, that's okay. The payment will error out and you can add the customer and try again.
The payment amount will default to the balance due on the transaction. If the customer has less store credit than that, you will need to adjust the payment amount and pay the balance of the transaction on another payment method (just like you would handle any other multi-tender transaction). Again, if you try to complete the transaction and use more store credit than the customer has, the payment will error out. You can void the payment, add the correct amount, and try again.