If you already have  Store Credit for particular customer so now you can use it for the same customer by applying Store Credit to Transaction.

In first step you need add at least one item to transaction, and from from left bottom corner add customer to transaction.

Click on the shopping cart drop-down, select the Store Credit Lookup mode.


You can search for a store credit using the scan panel as well as the magnifier for the advanced search.

Select the same customer as you already have  and Apply Store Credit To Transaction. 

Following window must appear.

You can change Amount to Apply, after click on Apply to Transaction , then click on Pay and Payments window will be shown as below.

Click on Done and you will receive receipt print out. 

Related article: 
Creating Store Credits