Lower heat to a simmer, then cover, and let simmer for 10 minutes, stirring occasionally.
Remove from heat, mash the grapes.
Put mashed fruit in a jelly bag.
Let drip for 2 to 4 hours, or overnight.
Measure how much juice you got. You need 1 litre (4 cups / 32 oz) of juice. If you didn't end up with that, see notes below.
Put the juice in a pot.
LIQUID SWEETENERS (HONEY, AGAVE, LIQUID STEVIA, ETC): See special directions below before proceeding further.
DRY SWEETENERS (SUGAR, SPLENDA, POWDERED STEVIA, ETC): Mix the pectin powder with the sugar in a small bowl or a measuring cup, set aside. Add the lemon juice and the calcium water to the pot. Bring pot contents to a boil, then add pectin mixture.
ALL SWEETENERS: When pot returns to a boil, let boil for 2 minutes, stirring constantly.
Skim surface scum off OR if desired strain again one last time through a jelly bag.
Ladle into 125 ml (4 oz) or quarter-litre (1/2 US pint / 8 oz) jars.
Leave 1 cm (1/4 inch) headspace.
Debubble, adjust headspace if needed.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner.
Process jars for 10 minutes; increase time as needed for your altitude.