Use a chromatic tuner or tuning app
Tune the strings to G, D, A, E from lowest-pitched pair to highest-pitched pair
Tune the lowest pair of strings to G3
Tune the next pair to D4
Tune the next pair to A4
Tune the highest pair to E5
Pluck one string at a time while adjusting its tuning peg
Turn the peg slowly until the tuner shows the correct pitch
Tune both strings in each pair to the same note
Check the tuning by playing double stops and open strings
Retune after playing, since mandolins go out of tune easily