Open the mobile app,
Switch to the 'Profile' tab,
Find the 'Fatboxes' tab and click on it,
Choose a Fatbox to buy and click on it,
Switch to the 'Inventory' tab,
Open the 'Lootboxes' section and click to open the lootbox that was dropped from the Fatbox.
If you have any questions regarding the contents of the Fatbox, please refer to this article.