If your recipe called for lemon juice and you forgot to put it in, your mixture will not be acid enough for safe canning. You have to open the jars and put the mixture into a sauce pan. (If you made the jam or jelly recently and you carefully remove the lids without damaging them, you can re-use the same lids.) Add the missing lemon juice and stir well. Bring the mixture to a good boil. Boil and stir for 2 minutes. Jar and process according to the recipe sheet that comes with Pomona’s Pectin.