Place the plums in a clean paper bag. Any paper bag will do, but it should be empty of all contents except plums. When plums (and other fruit) ripen, they emit ethylene. Placing them in a paper bag with the top folded over keeps the gas close to the plums, speeding up the ripening process.
Store the bag at room temperature. The plums will ripen best when kept at a temperature between 68 and 77 degrees Fahrenheit. Keep them at this temperature until they become fully ripe.
Enjoy the ripe plums. You can eat or cook the plums as soon as they’re ripe. To halt the ripening process and preserve them for a bit longer, store them in your refrigerator’s crisper