Refund of VAT from EU purchase

Another daft question i'm afraid...
I bought an item in the EU, and paid the VAT bringing it to the UK, via FedEx.
Now i'd like to return the item, and would obviously like to reclaim the VAT (less FedEx's fee)
I'm pretty sure i've done this before, but i can't find a 'how-to' guide anywhere. FedEx's explanation on their website is less-than-feeble:- https://www.fedex.com/en-gb/custome...toms/return-shipment-refund-duties-taxes.html.
can anyone explain in steps what i need to do, and what proof i need to keep hold of to prove i have resent the item to the EU-based vendor?
Many thanks indeed,
John.