Firstly, welcome to Kelly Hall via Boglovin.
Home grown and most bought plums seem to be big enough to stone by hand simply but cutting them in half, along their slight crease, twist off and top bit and then a slight fiddle to get the stone out the bottom bit.
Wild plums, such as the yellow and red ones that grow quite well along hedgerows around us, are sometimes a bit more fiddly. When stoning them, I reach for my mother-in-law's 'vintage' plum stoner!
It is a beautiful item in its own right and takes pride of place on my Welsh dresser in the dining room.
The plums need to be just ripe or slightly under ripe but not too much: