Although it's always best to use fresh berries when possible, jam can easily be made from frozen strawberries as well. It's best to start the jam-making process with strawberries that have already been defrosted.
Add strawberries, cup lemon juice and granulated sugar to a deep, heavy pot. For every quart of frozen berries, use 1/8th cup of lemon juice and about 4 cups of granulated sugar. Stir together until the mixture begins to come together.
Turn the burner up to the highest level of heat that it can reach. Continue stirring.
Continue stirring the mixture occasionally. The strawberries should start to soften and take on a liquid form. At this stage, add the pectin. Use the amount recommended on the package for the amount of berries you've cooked.
Bring jam to a rolling boil and cook for one full minute. Add a pat or two of butter and stir into jam. Turn the heat off.
Funnel jam into prepared home canning jars and can in your usual manner.