Put a small plate or bowl in the freezer.
Prepare your canning jars (wash, rinse, put on a sheet pan in a 200℉ oven) and canning pot.
Prepare a pot of boiling water for peeling.
To peel the peaches, working in batches of 2-3 peaches at a time. Cut an X into the bottom of the peach and blanch in boiling water for 30-60 seconds, remove to a bowl of cool water, and peel. Remove the pits and roughly chop the peaches. Put into a colander to drain while dealing with the rest of the peaches.
Once you've peeled, pitted, and kinda drained all the peaches, add to a 4qt pot.
Add the lemon zest (¾ tsp), lemon juice (3 tbsp), and sugar (1000 grams) [SEE NOTE], and stir to combine. Let sit for 10-15 minutes to break down the peaches.
While the peaches are macerating, put the jalapeño peppers into your food processor and mince, scraping down the sides as necessary. (Note: Adding ¼-cup of the peach puree to the food processor may help.)
Add the jalapeño peppers to the peaches.
Add the pectin and stir to combine.
If you want a smoother jam, you can either use an immersion blender before heating or a potato-masher while heating.
Heat over high heat, stirring occasionally to avoid burning on the bottom of the pot. Skim off and discard the foam.
Once at a boil, boil for 5-10 minutes. A candy thermometer should reach 220℉. If you don't have a candy thermometer, put a small portion on the plate you put in the freezer earlier -- this should cool it fast and you can check consistency and taste. Should wrinkle up as you push on it.
Carefully ladle the jam into the prepared jars. Wipe the rims and put lid and ring finger-tight.
Using canning tongs, add to canning pot and process in water-bath for 6-8 minutes, taking care to make sure jars are fully submerged.
Using the canning tongs again, remove to a towel and let cool until sealed.