You can find the original recipe here, but I have condensed it down in the following recipe to make it easier to read and follow.
Adapted from http://pickyourown.org/cherrybutter.php
4 or 5 Quarts Pitted Cherries (fresh or frozen work)
Fruit Juice (1 cup per pound of cherries)
Cinnamon and Nutmeg, if you prefer
Half-Pint Jars and Lids for Canning (2 quarts of cherries produced 2 half-pint jars of cherry butter)
1. Wash, de-stem and pit all cherries.
2. Put cherries in a large, heavy pot with fruit juice. (If you dont have a scale to measure how many pounds of cherries you have, add enough juice to almost cover all the cherries.)
3. Bring pot to a boil, then lower to a simmer, cover with lid slightly ajar, and simmer for 1.5 to 2 hours, until cherries are very tender.
4. After cherries are tender, run them through a food mill placed over another pot.
5. Add sugar and spices, if you prefer. The amount of sugar you add depends on your preference. I added 1/3 cup sugar to 2 quarts of cherries and felt that it was too sweet. You don't have to add any sugar if you don't want to. Taste the pulp and determine the amount of sugar you need from there.
6. Bring pulp to a simmer, cover with lid slightly ajar, and reduce pulp to consistency you like.
7. Wash jars in hot, soapy water. Leave jars in water until ready to use.
8. Put jar lids in a saucepan and add enough water to cover lids by 1/2 inch. Bring to a simmer.
9. Once oven is ready, ladle cherry butter into jars. Using a plastic cooking utensil handle or knife, push handle through butter to release any air pockets.
10. Run damp rag around the edge of the jars. Take a lid from the saucepan and place squarely on jar. Tighten with jar ring until just tight.
11. Place jars in waterbath 20 minutes.
12. Remove and let sit on counter until cooled and the jars seal (the little dimple on top of the jar lid will make a clicking sound and remain depressed when sealed.