How can I tune all strings on a guitar to ensure they are in harmony with each other?

To tune all strings on a guitar to ensure they are in harmony with each other, use a guitar tuner or a tuning app to adjust each string to the correct pitch. Start with the low E string and tune it to E, then tune the A, D, G, B, and high E strings accordingly. Check the tuning by playing chords and adjusting as needed until all strings sound in harmony with each other.