If you have never made jam before I suggest familiarizing yourself with the process. Here is a pretty good primer
First, wash and pit the cherries. This is infinitely easier with the use of a cherry pitter-- I highly recommend the old-fashion-y one I used, you can order it here.
Continue cooking until the fruit-sugar mixture starts to gel and sheet off of the spoon, looking jammy, and thick. If foam forms, skim it off, and stir and stir, it would be a drag for it to burn at this point, wouldn't it?
When you think it's about jam, turn off the heat, and get one of the frozen plates. Spoon on a good dab, and return it to the freezer for a couple of minutes to chill. If you can push a line into it, and it doesn't run back together, "Its Jam". If it doesn't, cook it some more, and repeat.
Carefully spoon the hot jam into your adorable, warm, sterilized canning jars, wipe off any drips on the top, and top with the disk and loosely screw on the band.
Prepare a large pot of boiling water, with a rack or steamer basket, or just a couple of extra canning rings. Carefully lower the jars of jam into the boiling water and process for 10 minutes. Lift out, cool well.
If you are making a small amount of jam, (like one or two jars) and intend to finish eating it right away, you can just refrigerate it without bothering about the water bath, but do still sterilize the jars before.