Using credits on iTunes
How (please be very specific) do I use credits (not credit cards) to purchase songs on iTunes
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Here are the instructions on how to use iTunes Gift Cards (aka "iTunes credit"):
https://support.apple.com/en-us/HT201209
That page gives details on multiple ways to redeem Gift Cards, including from your iPhone, computer or if someone sent you a gift in an email. And finally, there is information near the bottom in case nothing is working and you can't redeem a code.
Good luck ☺️