Start by sifting the powdered sugar into a bowl through a large metal strainer. Stirring the confectioners sugar with a spoon is a lot less messy than shaking the strainer or tapping it against your hand. This breaks up any lumps and the icing sugar is nice and airy. This will make kneading later on easier and you will avoid having lumps in the fondant.
When you see that all the agar powder is dissolved (it will take probably around 10 minutes) take the pan off the heat. Some grains that were stuck to the side of my pan didn't dissolve, but I didn't worry about those. Now, add the glucose/corn syrup and shortening. Stir the mixture until all of the shortening dissolves. Stir in glycerin and flavoring.
Put half of the sifted powdered sugar into another bowl. Make a well and pour in the agar mixture. Stir together with a spatula as much as possible, adding more icing sugar when needed. When you can't stir anymore start kneading in the powdered sugar as needed by hand. It takes me usually almost 4.5 cups icing sugar to get the right fondant consistency. Form the fondant into a ball.