Begin by finely chopping the Datil peppers and removing the seeds if you prefer a milder jelly.
In a medium-sized saucepan, combine the chopped Datil peppers, granulated sugar, and apple cider vinegar.
Put the saucepan on medium-high heat and gently stir the mixture until it reaches a boiling point.
Once boiling, add the liquid pectin and continue stirring.
Insert a candy thermometer and cook until the mixture reaches 220°F (104°C). This may take about 15-20 minutes.
If you prefer a more vibrant color, add a few drops of red food coloring at this stage.
Remove the saucepan from the heat and skim off any foam from the top.
Carefully pour the hot Datil pepper jelly into sterilized canning jars, leaving about 1/4-inch of headspace.
Use lids and rings to seal the jars, and process them in a boiling water bath for 10 minutes.
Let the jelly cool and solidify naturally at room temperature.