To redeem a Hay Day gift card, you need to follow these steps:

Launch the game and wait for it to load.
Visit the farm of a friend by tapping the Social button on the bottom right-hand corner of the screen.
Double-tap on the envelope that appears on the crate they filled or the plant they revived to send a thank you letter.
Gift cards can be found in thank you letters that players send after other farmers helped their Farm with completing Boat Orders, Truck Orders, serving Town visitors, reviving dry trees and bushes. You can only receive up to 7 Gift Cards per day, and hold a maximum of 40 Gift Cards at a time.
Once you have a gift card, you can use it to purchase Mystery packages that can contain diamonds or Exclusive Decorations from the Gift Catalogue. The catalogue can be accessed via mailbox, which is located beside the Roadside Shop. It is unlockable at Farm level 15, along with Alfred the postman.
To redeem the gift card, enter your Hay Day users ID and select the game server. Choose the recharge amount that matches your cards value, enter your email, and then enter the code youve received.