You'll be redirected to Roblox's gift cards page. Tap on Redeem, located at the very top of the page. Enter the empty Code field with your Roblox gift card's code, then tap Redeem to finish.

If you purchase the gift card directly from Roblox.com, you are able to load a custom amount. ASDA sells physical gift cards in the amounts of £10, £20 and £50.