Combine the sugar, glucose syrup, and water in a saucepan. Bring to a boil, stirring constantly.
Stop stirring. Cook to 117°C/243°F.
Pour the hot mixture onto a marble slab that has been lightly splashed with cold water.
Sprinkle cold water lightly on top of the syrup. Allow to cool to 50°C/120°F.
If using premade fondant to seed the syrup, place it on top of the syrup.
Using a scraper, agitate the mixture until the fondant crystallizes completely, turning it until it is a short-textured mass. This will take about 8 minutes with the added premade fondant and about 20 minutes without it.
Store the fondant in an airtight container overnight to ripen.