Wash the Granny Smith apples thoroughly to remove any wax or residue from the skin.
Dry the apples extremely well with a clean kitchen towel or paper towels. The apples should be completely dry so the candy coating sticks properly.
Remove the stems and firmly insert a candy apple stick into the top of each apple.
Line a baking sheet or countertop with parchment paper. Lightly grease the parchment if desired to prevent sticking.
Add the sugar, corn syrup, water, and red food coloring to a medium heavy-bottom saucepan.
Stir gently just until the ingredients are combined.
Place the saucepan over medium to medium-high heat and bring the mixture to a boil.
Once the mixture begins boiling, stop stirring to help prevent the sugar from crystallizing.
Attach a candy thermometer and continue cooking until the mixture reaches 300–310°F, or the hard-crack stage.
Immediately remove the saucepan from the heat once it reaches temperature.
Allow the bubbling to settle for about 20–30 seconds.
Working carefully and quickly, tilt the saucepan slightly and dip one apple at a time into the hot candy mixture.
Rotate the apple until completely coated, then lift it from the saucepan and allow the excess candy to drip off.
Place the coated apple onto the prepared parchment paper.
Repeat with the remaining apples. If the candy coating becomes too thick while dipping, briefly return the saucepan to low heat until it loosens again.
Allow the candied apples to sit at room temperature for about 10–15 minutes, or until the candy shell is completely hardened.