Add the berries to a saucepan over medium-low heat.
As the berries warm, they’ll begin releasing their juices. Stir occasionally and gently mash larger berries with a spoon or masher.
Add vanilla and sweetener if needed. ( I usually don't add this since it is sweet from the fruit).
Simmer for about 8–10 minutes until the fruit becomes soft and jammy.
Remove from heat and let cool completely. The mixture will continue thickening as it cools. I put it in the refrigerator to help it thicken.