You can use store-bought ginger instead of making your own, but homemade candied ginger isn’t hard to make and it has a couple of advantages: feisty ginger flavor and a less sticky texture that’s easier to chop.
Peel and slice the ginger into 1/8-inch-thick rounds. In a medium saucepan, mix 2 cups of the sugar with 3 cups water and bring to a boil over high heat. When the sugar is boiling, add the ginger and simmer on medium heat until the ginger is very soft, 45 minutes to 1 hour. If the water evaporates before the ginger is soft, add more water.
Line a rimmed baking sheet with parchment. Spread the remaining 1 cup sugar on the parchment. With a slotted spoon, transfer the ginger to the sugared baking sheet. Toss the ginger pieces in the sugar and spread them out in one layer. Let dry for at least 2 hours or overnight.