Put on medium heat. Constantly stirring, bring to a boil and cook for 1 minute until the agar dissolves and the liquid turns into a transparent paste.
If you use gelatin, prepare it according to the instructions and dissolve it in a syrup of 500 ml of water and 200 g of sugar ( the temperature of the syrup is not higher than 60 degrees).
If potato starch, then first it must be diluted in a small amount of cold water. Mix the remaining 500 ml of water with sugar. Bring the syrup to a boil, pour in the diluted starch and cook over low heat, stirring, until thickened. Cover the pan with a lid or cling film and cool.