Place plums in crockpot with a little water.
If desired, add a little more water to thin to your desired consistency, and blend in well. The best way I can describe the consistency is a velvety spread that spreads quite easily, but is not at all runny.
Water ratio - I have had plums where I needed to add more water at the end, and some where I literally was trying to steam off every bit of liquid there was on there because they were so juicy! So, this varies a ton. My rule of thumb is put in just enough water to start so that you are not worried of scorching skins during the initial breakdown process. And then just go from there. If it seems like there is too much liquid, I just remove the lid a few times during cooking, let some steam escape, and pour off all of the condensation that's on the lid.
Lazy upfront style - I have totally done this before without initially removing any of the pits or halving the plums. Literally just threw all the plums in the crock pot with the spices. And guess what? It worked! I mean obviously, still remove the pits from the broken-down plums before blending. I'm not sure which method I prefer. Depends if I want more work initially or at the end.
CANNING! A lot of people have asked about this. Please refer to the comments for advice. I myself am not much of a canner, and have canned this once, but not enough that I feel adequate to provide detailed instructions yet. To maintain vibrancy and flavor, adding a little bit of lemon is good (although not sure if necessary). A standard 10 minute water bath should do it for processing. However, once again, read the comments and do a little more research. Once I have had the chance to can this a few more times and am 100% sure on the process, I will update the recipe.
Dress it Down:
To keep it simple, leave out the spices to enjoy pure plum bliss!