Cube the watermelon, ensuring to discard the rind and only use the flesh of the watermelon.
Mash the watermelon cubes with a fork or masher in a large pot.
Add sugar, lemon juice, and a pinch of salt to the pot.
Stir the mixture and bring it to a simmer over medium heat.
Cook the watermelon down for 20-30 minutes, stirring occasionally.
Optional: Add 1-2 cups of strawberries at this stage, blending them with the watermelon or adding them at the final stage of cooking for a unique fruit jam.
Add pectin and stir until fully dissolved.
Simmer for another 15-20 minutes, stirring occasionally.
To test for thickness, spoon some jam onto a cold plate, let it sit for a minute, then tilt the plate to see if the jam holds its shape.
Once thickened, remove from heat.
Pour the jam into sterilized jars, leaving some space at the top.
Seal the jars and allow them to cool to room temperature.