Take a pan, add the frozen mixed berries to it, let it thaw on the pan, once its ready let it simmer for a few minutes until the berries go soft.
Once the berries are softened, you can gently mash it or break it down with the back of your spatula. Let it simmer for 5 minutes on low to medium flame. Remember high flame will caramelize it and burn it quicker not bringing out the sweetness as it should.
When the berries are slightly thickened, add the chia seeds and mix it in. The jam will thicken once the chia seeds added.
Finally add the sweetener in this recipe I have used honey and bit of stevia extract powder. You can use white sugar or brown sugar or maple syrup.
If you are using sugar ( white or brown ) in the recipe, You can add it to the berries at step 2 do cook for a few more minutes until the sugar dissolves.