In a large mixing bowl, mash the sliced strawberries using a potato masher or blender until you achieve a chunky consistency. Set aside.
In a large pot, combine the mashed strawberries, sugar, and lemon juice. Stir well to dissolve the sugar.
Place the pot on the stove over medium-high heat and bring the mixture to a rolling boil, stirring constantly.
Once boiling, quickly stir in the pectin. Continue to boil vigorously for 1-2 minutes, stirring constantly until the mixture thickens.
To test if the jam is done, place a spoonful of jam on a cold plate and let it sit for a minute. If it holds its shape and does not run, it’s ready. If not, boil for an additional minute and retest.
Remove the pot from heat. Using a ladle and jar funnel, pour the jam into clean, sterilized canning jars, leaving about ¼ inch of headspace at the top. Wipe the rims of the jars with a clean cloth to remove any residue.
Place lids on the jars and screw on the metal bands until they are fingertip-tight.
For longer shelf life, process the jars in a boiling water bath for 10 minutes. Ensure jars are fully covered with water and allow to cool completely before storing.