Open the Google Play app.
At the top right, tap the profile icon:
Tap Payments & subscriptions:
Redeem gift code:
Enter the code.
If redeem succeeds, you will see that you have a free trial for 90 days:
Open the app and go to subscription screen as usual. Choose annual plan (as our coupon code is for yearly plan):
You will now see that you have a free trial for 90 days (from the promotion code):
Subscribe and you will have a 90 day free trial, and will not be charged any money.
If you don’t want to keep the app, please cancel the subscription before the free trial ends. Information on how to do that is found inside the app (“My profile” section).