Are you the seller or the buyer?
If you are the buyer, contact your seller - it us up to them to put in the claim. You would need to return the item in the box you received it in, the seller would need to reimburse you for the item and the shipping and then the rest is up to the seller.
If you are the seller and have received your item back from your customer, you need to get a claim form - the USPS has made it very difficult to get the form online and put in a claim, so go to your local post office and get a form from them. Fill it out, give the form and the damaged merchandise with the original shipping box to the postal worker, then wait 4-6 weeks for the post office to reimburse you. Give them pictures of the item as you listed it on Etsy to show its original condition to back up your claim. Hope that helps. I've gone through it 3 times now.
Linda