To perform Engoldenation you need to have at least 10 of one card.
To check your card collection click "Collect" from the main menu.
If you own multiple of one card it will be shown in a white circle on top of the card with how many you currently own. Open sacks from the collection screen to gain more cards until you reach 10 of one card, then click on that card and it should say "Engoldenate" just above the build card button, click this button and confirm the removal of 10 cards. The achievement will popup right after confirmation.