You're going to make a salt water bath using a ratio of one teaspoon of kosher salt for every cup of water. For a large apple, you'll need one cup of salt water.
Then, peel the apple and cut it into 1/4-inch cubes. Add the diced apples to the salt water to soak for about 10 minutes, drain well, and mix into your favorite egg salad recipe.
If you have leftovers, store it tightly covered and refrigerated for up to five days.