I live in Sacramento, CA and have a wonderful old Santa Rosa (I think) plum tree. Can you please give me advice about when and how to prune?
When and How to Prune Plum Tree
You should prune a plum tree when the buds are breaking. If you prune the tree prior to bud break you will cause the tree to come out of dormancy to soon. A Japanese plum should be pruned in an upside down umbrella shape with three to four main limbs. A prune plum can be pruned leaving several scaffold limbs in a Christmas tree shape. Longer limbs at the bottom opposite of each other at different heights.
This article should help: https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-pruning.htm