In a bowl, roughly mash your berries with a fork, pastry cutter or potato masher, or simply squeeze them with your fingers if they're soft enough that you can do that, right into a large, heavy skillet. Set it over medium-high heat and stir in the sugar and lemon juice.

Cook, stirring often, until the mixture bubbles and the fruit softens. Simmer for 20-30 minutes, or until it thickens enough that your spoon leaves a trail through the jam. Remove from heat, scrape into a jar or bowl and cool completely.