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 Roblox is not downloading or installing on your Windows 11/10 PC, restart your PC and if that doesn't help, use the fixes discussed here.

The Roblox help page states that purchases may be delayed, joining games may be unsuccessful, and there could be lag.