Return policies are very industry and location specific. Following are some main return issues you need to define for your store.
Are all your products returnable or do some restrictions apply?
Set the time limit for returns.
HOW do customers return items?
Tell the customer how to return items, what documentation they need etc.
Are original shipping rates refundable? Does customer pay for shipping the return?
Set how customer is credited for returned item.
If you have brick and mortar locations, can customers return items there. Provide locations, directions and hours of operation.
Do items need to be in original packaging for returns?