Once you get your code, you go to www.minecraft.net/en-us/redeem sign into your Microsoft acout (the same one that your minecraft is linked to) then scroll to the redeem box put in the code and redeem. It could take a few minutes to appear on your account.
@@mr.gamingaquaticthe fun thing is that from my expirience, the redeem sections for the capes are only on the US website, i tried on my native site but with no result. so i had to fool the site into getting me on the US site and redeemed them there.