When you issue store credit to someone, they will be given a receipt with a barcode and a store credit number. You can use either of these from the payment screen to use the store credit.
At the payment screen, either scan in the barcode on the receipt, or type in the store credit number and then click on the store credit button. This will add the store credit as a tender, if this does not cover the entire amount owed use another tender to complete the sale.
If the store credit covers more the the amount owed, you can do one of two things. Complete the sale and give the rest of the store credit back as cash. To do this you will just finish the sale like normal. The other option is to generate a new store credit slip for the remaining amount. To generate a new receipt, click the Store Credit button again. A window will pop up showing the amount that the store credit is for, click Accept and it will close the sale and print both a sales receipt and a new store credit slip.