To settle new strings: tune to pitch, gently lift each string slightly along its length, retune, and repeat until the tuning holds. Don't pull aggressively or make sharp bends, which can damage the string.
Set witness points so the string makes firm contact with the nut and bridge saddle. It's one of the most overlooked steps, and it improves tuning stability, intonation and sustain.
Make sure the strings are wound neatly around the tuning posts. If tuning problems continue after the strings settle, check the nut slots, bridge saddles, tuners and overall setup. Persistent problems usually point to setup or hardware rather than the strings.