Update: I've revised this, possibly the most popular umeboshi recipe in English online, to include some key troubleshooting notes. Originally published June 18, 2009. My mom has been making a batch of umeboshi every year since, and I've also added some more notes from her.
My mother came for a visit this week, bringing along a pot of her homemade umeboshi. I asked her to tell me how she makes them; not only did she write it down for me, she even had pictures she'd taken of her attempts in the past couple of years! So, here is my mom's version of how to make homemade umeboshi. I've freely translated her Japanese explanation to English.
My mother [my grandmother - maki] used to make umeboshi every year. When I lived in New York, I was too busy working to do much cooking, let alone umeboshi! But now that I am retired, I'm trying to remember how to do things the old way. Homemade umeboshi is so much more delicious than store bought, so they are worth the effort.