Place the berries and lemon juice in a fairly large pan, then heat for around 10 minutes, just until the berries are nice and soft. If you have any large lumps, just squidge them with a fork.
Add the chia seeds and continue to cook for a further 15 minutes or so until a thick jam consistency forms. Taste and adjust the sweetness if necessary. Remove from the heat, and allow the jam cool completely.
Pour into a jar(s) with a lid. It won't last as long as shop-bought jam, but if you store it in the fridge, it should last for a week.