Put as many cherries as you've managed to pick into a large pot, add half a cup to a cup of water (less than a cup if you have under 8 cups of berries; a cup if it's more) and bring to a simmer over medium-high heat. Cook until the cherries soften and start to release their juices, mashing occasionally with a potato masher.
Strain the mixture through a cheesecloth-lined sieve set over a bowl - or use a jelly bag if you have one. Leave it if you want a clear jelly, or swirl a spoon around in the sieve to coax out as much juice as you can. (This is what I do.) When you get out as much as you can, toss the sludge with all the pits in it, and put the juice back into the pot.
Measure out about as much sugar as you have juice and set it aside. Add about 1 Tbsp. lemon juice per 2 cups of juice to the juice, and shake in some packaged pectin - I had about 5 cups of juice and used about half a package. (Most recipes call for a packet for 6 cups of juice; you can totally guesstimate here.) Bring the juice-pectin mixture to a full rolling boil, then stir in the sugar. Bring it back to a full, hard boil for 2 full minutes - this means a rolling boil you can't stir down. Remove from the heat and skim any foam off the surface. Ladle into hot, clean jars, seal and cool.
Makes as much as you like.