I would say first have them be quipped with the materials that they need, such as tuner, metronome, books to learn from, and music stand. Second, tell them to try to practice everyday, even if it’s only for 10 minutes, little bit goes a long way. Third, learn a piece and practice it, once they learn a second piece, always go back to the earlier piece and keep refreshing their techniques. Always warm up with a scale or two when practicing, and once they are able, study etudes.
Last and not least, always practice with a metronome.