Add sugar, lemon juice, and water to peach slices. Stir to combine.
Cook over medium-low heat for about 45-60 minutes, or until peach slices have given off a lot of juice and have cooked down to a thick syrup.
Once slices have cooked down, you can puree it or leave the syrup chunky.
Notes
I leave the skins on the peaches, and they cook down and help to thicken up the syrup a little. Feel free to peel the peaches first if you prefer thinner syrup.