You would create an RMA one  (1) line for one invoice and one (1) line for the other. The restocking fee should auto populate as long as you have the items setup for restocking.

I created a sales order titled RMA. Do I then go into Invoice Data Entry and create a credit memo? How do I issue the payment?

Or should I not create a sales order using the letters RMA?

RMA is an optional module for Sage 100.  If you have it, use it. 

If you don't have it, when you receive the items just go straight to the Credit Memo (and add negative misc charge lines for the restocking fees to reduce the credit amount).

I don't have the option to do what it states to do in the knowledge base article 

Thank you! I don't have RMA.

How do I issue payment?

https://help-sage100.na.sage.com/2020/index.htm#Subsystems/AR/ARPROCED/REFUND_A_CUSTOMER.htm

I usually recommend using a zero $ cash receipt for step 2, but do whatever you wish.

So I need to create a cash clearing account, create the credit memo, the create a refund check.