Smash the thawed frozen berries, or fresh berries, with a potato masher or a fork.
Add the chia seeds: if the mashed berries are thicker in texture (blackcurrants, blueberries), add 1 tablespoon. If the mashed berries are juicier (strawberries, raspberries) add 2 tablespoons.
Try for taste, and add the lemon juice and sweetener as needed. If adding more than two tablespoons of liquid, increase the amount of chia seeds.
The jam can be eaten straight away, but it will get thicker after setting in the fridge for a few hours.
Notes
Nutritional Information is calculated based on raspberry jam, made with 1 1/2 tablespoon of chia seeds. All berries are low in carbs, but if you are calculating your macros check your chosen berry nutrition facts.