Determine the comfortable range for the singer. This can be done through flexibility exercises, scales and singing. A vocal teacher can do this fairly quickly.
Sing the chosen song. Are the high notes or the low notes out of range? If the high notes are out of range, you will need to lower the key of the song. If the low notes are out of range, raise the key. It is rare for the key of a vocal song to be altered more than one whole step (e.g. Bb to C).
Change the key. How you do this depends on your skills. An experienced accompanist can often transpose the keys by ear. Some accompanists may require the music to be rewritten in the new key. If you are using computer software to write the music, you can do this fairly easily. Follow a strict formula to raise or lower the key. For example, if you want to raise the key from Eb to F, every note in the song must be raised one whole step (C to D, F to G, Bb to C, etc.).
Practice the song in the new key. This takes extensive training if you are transposing the music on piano. Make additional adjustments if needed.