Agar-agar, if time permits, I soak for several hours in a small amount of water, more often at night. I think it dissolves faster in boiling liquid.
Pour sugar into the milk and put it on the fire. When the milk is heated, add the agar and bring the milk to a boil. Cook, stirring, for a few minutes, until the agar-agar completely dissolves. This is the difference between agar-agar and gelatin, which in no case can be boiled.