Decide on the spirt you want to use. It could be a white spirit (gin, vodka) or a malt based spirit (whiskey, brandy), or a rice based spirit (sake, soju, shochu), or a spiced spirit (rum). Generally you are going to choose a spirit that you enjoy from the outset.
Decide on the sweetness level you want your Umeshu to be at. Japanese method is to add equal weight (1:1) Ume to rock Sugar. Thats way too sweet for me, so I reduce the sugar right down to 20-30% of fruit weight. Keep in mind you can always add sugar later, but you cant take out.
Prepare Ume - rinse Ume in cold water. Remove the small black fruit cap with a toothpick. Then dry fruit on a towel or tea towel
In clean dry dishwasher washed 1-2 size jars are the best- add Ume, then rock sugar, then fill to top with the spirit. You’ll likely be using 700 -1litre of spirit. Put lid on and leave for aprox 6-12 months before drinking. Record your ratios so once you start tasting, you can adjust.
Umeshu will just get better with age. The liquid will darken in colour which is part of the "aging" process. 20 year old Umeshu is usually opened with great ceremony!