To redeem Discord Nitro Classic Monthly, please follow the steps below:

Log in to your Discord account on the desktop or browser version of the app.
Click on the cog wheel icon next to your username to open up User Settings.
Then click on the Subscriptions tab in the left side bar.
If you have a Nitro Classic Monthly subscription credit, you can activate it by following the steps below:
When your current Nitro Classic Monthly subscription has expired or is removed, the subscription credit for the gift will apply when you activate that matching subscription on the desktop/browser Discord app.
Start by heading to the Subscription tab and pressing the Subscribe button for the Nitro Classic Monthly subscription type.
A new window will pop up where you will need to select the matching plan for the subscription.
If your Discord account does not have your payment information, youll be asked to fill out your payment details. (Note: You will not be charged unless the subscription renews)
Once youve finished submitting that information, youll then see a Subscription Credit applied message as you are reviewing the purchase details for activating the subscription.
After you confirm your activation, your subscription credit will then be activated!