How to redeem itunes gift card for Clash of Clans Gems?

My son has been playing clash of clans for a while now and was always asking me to use my card so that he could buy clash of clans gems, i always refused because i don't agree with spending money on a game like this. Clearly so determined he said he found a way of getting free clash of clans gems easily and showed me this video:

He know has a few Itunes gift cards and neither of us know how to redeem them for the free clash of clans gems. Can anyone help with this? I have added the video link he used because i have no idea about Iphones or any of this technical stuff… I still have my Nokia C2-00. Does the job.

Hi, maybe this could help
Install the latest version of iTunes. You can download iTunes (it's free) from the Download iTunes webpage. If iTunes is already installed on your computer, check your version and update if necessary.
Open iTunes.
In the Source list (on the left side of the iTunes window), click iTunes Store.
In the upper-right corner of the iTunes Store window, click the Sign In button. If you are already signed in, skip to step 6.
Sign in to the iTunes Store using your iTunes account information.
In the QUICK LINKS window in the right-hand column, click Redeem.
In the resulting screen, enter your 16-digit code which starts with an "X". (If you have an iTunes Gift Card, this is printed on the back of the card). Then, click Redeem.
Once you've successfully redeemed your code, the credit amount will appear next to your account name in the upper-right corner. Each time you purchase items, iTunes will deduct funds from the credit until it's depleted. If a pending purchase exceeds the credit amount, iTunes will ask you to enter a credit card number. You do not need to re-enter your gift card number to make multiple purchases.