Wash your strawberries well and clean the leaves of the tops. Place the strawberries in a big pot on a low-medium heat. Add in the sugar and lemon juice and let cook. Don't let it get too hot, you don't want it to be boiling over, just simmering, put it on a lower heat if you need to. Skim off the foam that forms at the top of the mixture, continue to do this until jam is reduced and foam no longer forms. You can also smash the strawberries from time to time so you don't have massive chunks. It can take 1-2 hours to cook.
Keep a bowl to the side of the pot for the foam skimmings, you can strain the foam off and it makes a really yummy strawberry syrup.