You need to open an Unpaid Item case, which you can do once a full 4 days since the sale has passed. The case can be closed after another 4 full days and the FVF will be credited back to your account (when you view the case details, it will say "awaiting payment", but once the 4 days have passed it will switch to "You can close this case now". The buyer can still pay during the 4 days the case is open, and will close automatically - without a FVF credit - if they pay by PayPal). 

If the case closes without payment, the buyer will receive a strike on their account, which may help prevent them from bidding on or buying items if the seller has blocks in place (to put blocks in place, go to My eBay > Account > Site Preferences > Buyer Requirements, and block buyers with two or more strikes in 12 months for the strictest setting).