To get the King Tut Mask item in Animal Crossing: New Horizons, you’ll have to discover the recipe.
To get the idea for the DIY, you’ll have to hit a Gold Nugget out of a rock using your shovel. Once you pick the nugget up off the ground, your character will think for a moment, and then get the idea to make the King Tut Mask.
It takes five Gold Nuggets to craft the mask.
Note that if you’ve already found Gold Nuggets from rocks before the update, you will not retroactively get the mask recipe. You will have to do it again to unlock the DIY.
If you’re not having any luck with the six rocks on your island, you can try traveling on a Mystery Island Tour. The rocks on these islands can drop Gold Nuggets, but it’s still a rare occurrence.
Once you get your mask, you can wear it, but keep in mind that doing this will cause your character to trip and fall.