To activate your compatible amiibo in-game, please head to the amiibo building in the Peach's Castle area (hub). This building will be located behind the castle.
You can access this by walking around, or taking the pipe to the right of the castle.
Once you entered the building, a new screen will display. You will be able to use any of the available amiibo to activate. This includes: Mario, Peach, Yoshi and Luigi. To add one amiibo in your collection, simply press A.
You will now be prompted to hold the amiibo to the NFC touchpoint. Place the amiibo as seen in the in-game prompt.
Then, you will receive a confirmation for your new reward. Here is an example for the Mario amiibo:
Your amiibo should now be available in your own in-game collection! Be aware that 4 new weapons can be unlocked through this feature and that you should receive one new exclusive weapon per character.
Please note that every figure of the same character (i.e. Mario Gold, Mario Silver or Super Mario) will unlock the same weapon.
For more information about the content, please visit this article HERE for a breakdown of the content available for each amiibo.
If you are unsure where to find the reward you unlocked, please check out this article.