For all marketplaces, you are essentially pulling up the order, in the upper right hand corner on the order page you will see "-Select an Action -" The list below instructs you on what the "Action" is called in the list so you know what you are looking for. Once you select it, hit "Go". 

Helpful facts: 

Sales tax: Always zero out sales tax on Amazon. On Walmart, RBO, eBay, we DO need to refund sales tax if it shows up on the order.  Make sure you are including it in the amount you are refunding. Match the order total. If you are doing a concession, it is up to you to figure out what the sales tax is for those marketplaces. CWA does display tax percentage on the main order page so you know what to recalculate it at if giving a concession.

Walmart, if the customer has a return request open - concessions are not an option. When you attempt a refund through the typical workflow you will notice a message at the top "Click here if you wish to complete the RMA on Walmart". Once you click the link, on the next screen you merely hit checkboxes to the left of what you want to refund and then hit "save" at the bottom. This is the only way to refund the customer, and will give the customer a 100% refund. They need to cancel their return request if they want to accept a concession. This is not a Sellercloud error, this is just how the Walmart website works now.

Helpful Info
"Issue Refund"
Be sure to zero out sales tax. Amazon will refund that on their own. Do not include it in refund amount.
"Refund Payment"
Please include order number in the required note field. Please calculate sales tax and include it in refund amount if sales tax was present and you are doing a partial refund. It is our responsibility to refund the sales tax. If for any reason it doesn't default to some sort of "Issue PayPal Refund" you might need to select it but I do not believe that you do need to do that.
"Issue Refund"
Concession isn't possible if return request is open. You can issue full refund still, select "Issue Refund", at the top of the page you will see a message stating an RMA is open and asking if you'd like to process it. Click the link. Check the box next to the product you wish to refund and hit "Complete RMA". If no RMA is present, it will be just the normal screen - select the product, make sure the product price and sales tax fields are filled in and hit save.
Suzanna or Chris needs to refund directly in PayPal, then Chris needs to be emailed to log the refund in SellerCloud.
Overstock Marketplace
Channel Advisor
Refund in Channel Advisor, then email Chris: order number & refund amount
Google Express
Channel Advisor
Refund in Channel Advisor, then email Chris: order number & refund amount
"Cancel/Issue Refund"
Chris has not tested this. You probably check a box next to a product, edit the dollar amount and hit some sort of save button. Grab Chris if you have trouble.