Follow these steps to redeem your Minecraft Minecoins:

  1.  Sign in to your Minecraft account in your browser.
  2. Go to the Redeem page, next to your username. 
  3. Scroll down until you find "Redeem Minecoins" and enter your code.
  4. The Minecraft Minecoins are added to your Minecraft account. 

Note: You can find the official FAQs here.