First go online to check your nearest participating store (link is for store locator) you’ll need to enter your postcode and tick the ‘Participate Free Cone Day’ box, then search

Once you’ve found a participating store, all you need to do is turn up between 12pm and 8pm and ask for your free cone - there's no code or voucher needed. It’s one scoop per cone (norm £2.50), choose any flavour.
Ben & Jerry's even says you can join the queue again if you're after another free scoop.