Before you begin, prepare calcium water.To do this, combine ½ teaspoon calcium powder (in the small packet in your box of Pomona’s pectin) with ½ cup water in a small, clear jar with a lid. Shake well.Extra calcium water should be stored in the refrigerator for future use.
Wash jars, lids, and bands. Place jars in canner, fill canner 2/3 full with water, bring to a boil. Turn off heat, cover, and keep jars in hot canner water until ready to use. Place lids in water in a small sauce pan; cover and heat to a low boil. Turn off heat and keep lids in hot water until ready to use.
Cut thin slices of unpeeled guava and put in sauce pan. Add water. Bring to a boil, turn down heat, and simmer covered for 5 minutes. Mash guava. Pour mashed fruit into a jelly bag and let drip into bowl until dripping stops. Gently squeeze jelly bag for last bit of juice.
Measure 4 cups juice into sauce pan.
Add calcium water and lemon juice (if needed), and mix well.
Measure sugar into a bowl. Thoroughly mix pectin powder into sugar. Set aside.
Bring juice mixture to a full boil. Add pectin-sugar mixture, stirring vigorously for 1 to 2 minutes to dissolve the pectin while the jelly comes back up to a boil. Once the jelly returns to a full boil, remove it from the heat.
Fill hot jars to ¼” of top. Wipe rims clean. Screw on 2-piece lids. Put filled jars in boiling water to cover. Boil 10 minutes (add 1 minute more for every 1,000 ft. above sea level). Remove from water. Let jars cool. Check seals; lids should be sucked down. Eat within 1 year. Lasts 3 weeks once opened.