There is no RMS setup for these features, but you must have relatively up-to-date NSc Sync and GWO software.
To enable the features navigate to your Web Store Manager (WSM) > Payments > Payment Gateways > select your gateway & Configure. If the options are available per the gateway you may simply select the options & Save.
Transactions going forward will include these options, unless you again disable the features in the same way.
To capture through RMS you only need to Tender as usual. You can decrease the value of the order on the POS and that value is all that's captured. You may also increase the value, but within the limits set forth by your payment gateway.
To return through RMS you only need to Recall > Recall for Return and Tender, just as you would if the customer were standing at the counter.