Combine the berries and sugar in a large, heavy-bottomed saucepan. Bring to a boil over medium-high heat, stirring constantly with a flat-bottomed wooden spoon. The mixture will be dry at first, but in a few minutes the berries will start releasing their juices. Stirring constantly prevents the sugar from sticking to the pot and burning before the juices start flowing.
Keep cooking and stirring until the mixture comes to a full boil and all the sugar is dissolved.
Adjust the heat to keep the fruit mixture bubbling steadily at a low boil and start timing. It will take about 20 to 25 minutes after it reaches a boil for the jam to cook, but it will depend on the temperature of your burner and the diameter of your saucepan.
Clip a candy thermometer to the outside of the pot, so the tip of the thermometer is not touching the bottom of the pot, but is about halfway into the depth of the jam.
Cook the jam until it reaches the jell stage - 220℉ (104.4℃). The jam temperature will hover around 210℉ (99℃) for about the first 15 minutes, and then will start slowly increasing toward 220℉. As it gets closer to temperature, the foam will subside and you'll notice a difference in the texture - it will get more syrupy and start plopping and splattering droplets a bit.
If you don't have a thermometer, you can test the jell stage by dropping a bit of jam onto a chilled plate and checking how firm it is.
When the jam reaches the jell stage, remove it from the heat and pour it immediately into sterilized canning jars, filling them up to ¼-inch from the top of the jar. Wipe the rims with a clean, damp cloth, add a hot snap lid to each jar, and then a screw band which you should just tighten finger-tight.
Set the jars on a towel to cool, undisturbed, or process the jars in a water bath for 10 minutes.
When cooled, check to make sure the jars are sealed - the snap lid should be sucked down tight and not move or pop when pressed.
Notes
A candy thermometer is the most accurate way to test if your jam has reached jell stage. It is an inexpensive investment, worth it for jam-making.Store any unsealed jars in the fridge and use up within 3 months.Unopened, sealed jars will keep, unrefrigerated, in a cool, dark place for up to 2 years, or longer.