Dice the watermelon into 1/4-inch pieces, making sure to remove all seeds as you go—this prevents any unwanted texture in your finished jam. Measure out all remaining ingredients and have them ready near your cooking station. This mise en place approach means you won't be scrambling to find ingredients once the mixture starts heating and thickening.

In a large heavy-bottomed pot (at least 4-5 quart capacity), combine the diced watermelon, sugar, and lemon juice. Stir well to combine, then bring the mixture to a rolling boil over medium-high heat, stirring occasionally to prevent sticking. This initial boiling helps the watermelon release its juices and allows the sugar to dissolve evenly, creating a more cohesive jam.

Once the mixture reaches a rolling boil, reduce the heat slightly to medium and sprinkle the pectin over the surface while stirring constantly for about 1-2 minutes until it's completely dissolved and no lumps remain. I find that stirring continuously prevents the pectin from clumping and ensures even distribution throughout the jam. Once the pectin is fully incorporated, return the mixture to a full rolling boil and stir in the salt and lemon zest.

Maintain a full, rolling boil for exactly 1 minute, stirring constantly—this brief intense heat activates the pectin's gelling properties. Then reduce the heat to medium-low and simmer gently for 20 minutes, stirring frequently to prevent the bottom from scorching and to encourage even cooking. The jam should darken slightly and become more viscous during this time. To test if it's ready, place a small spoonful on a cold plate; it should wrinkle slightly when you push it with your finger, indicating proper set.

Remove the pot from heat and let the jam rest for 2-3 minutes to allow any foam to settle. Carefully pour the hot jam into clean, sterilized jars, leaving about 1/4 inch of headspace. Wipe the rims clean, seal with lids, and let the jars cool completely at room temperature for at least 6-8 hours before moving them. The jam will continue to set as it cools, reaching its final texture by the next day.