Remove the red shiso leaves from their stalks, rinse, then thoroughly drain in a colander.
Lightly sprinkle with salt and let sit for about 10 minutes.
Once the leaves begin to wilt, tightly squeeze to press out their astringency. Repeat this process two more times.
Sprinkle on a small amount of ume vinegar to fix the color. Put the umeboshi in a jar, sprinkle the shiso leaves on top, then seal. Store until midsummer. Follow the instructions on these other recipes to see how to make umeboshi plums.
"How to Sun-Dry Umeboshi" (Recipe ID: 616150). This is the same method used for sun-drying white umeboshi, but it includes the red shiso leaves.
"How to Make Low-Sodium Umeboshi" (Recipe ID: 608314) and "Yukari Rice Topping Made with Red Shiso Leaves" (Recipe ID: 1725655).
"Our Family's Umeboshi with Bonito Flakes" (Recipe ID: 608205)
"Hijiki and Umeboshi Pasta" (Recipe ID: 621624)
Story Behind this Recipe
This is the pickling method I learned from my mother.
These are amounts of ingredients you need to make 2 kg of umeboshi. Feel free to add more red shiso, if you like.