Wash and hull the strawberries, then slice them into smaller pieces.
In a large pot, combine the strawberries, lemon juice, and sugar.
Stir the mixture and let it sit for about 10 minutes to release juices.
Over medium heat, bring the mixture to a boil, stirring frequently.
Once boiling, reduce heat and let it simmer for about 20 minutes.
If using pectin, add it according to package instructions during boiling.
Test the jam consistency by placing a spoonful on a cold plate.
Remove from heat and let it cool slightly.
Pour the jam into sterilized jars, leaving some space at the top.
Seal the jars tightly and let them cool completely at room temperature.