Website Activation:

1. Go to the Google Play website - here
2. Sign up or log in using the button in the top right corner.
3. Go to the redeem code page - here
4. Enter your code and click Redeem.

Mobile App Activation:

1. Open the Google Play app.
2. Sign up or log in using the button in the top right corner.
3. Tap your profile icon in the top right corner.
4. Go to Payments and subscriptions.
5. Select Redeem code.
6. Enter your code and click Redeem.