Add in the Pectin (Certo liquid pouch),, stir to mix through and then increase the heat a little (med-high), and let the fruit come to a full rolling boil. For a true rolling boil, the volume of the mixture will rise up, and there will be a mass of bubbles. See Photo 3. Another way you can tell is when you stir, if it stops boiling and moving vigorously, then it is not a rolling boil. Apart from testing, don't stir the mixture. Keep an eye on the temperature. You want it to reach 105°c/221°f for jam. (The sugar thermometer shows the different setting temperatures for different things, including jam). Once it reaches that temperature, remove from the heat.