Find your student's range. It's important to find the upper register for your student before you begin. Have your student sing a scale. Start playing the scale on middle C on your piano and have your student sing along. Play notes until he struggles. This is where you want to begin.
Instruct your student to ignore what they've been taught about singing properly. This means ignoring the rule about singing from the diaphragm. Singing falsetto is almost the opposite of proper singing. The notes should come from the upper throat. This will cause the outer edges of your student's vocal cords to vibrate, which causes a distinct rise in pitch. Ask your student to place his hand on his chest. If he feels it vibrating, he has not yet left his normal singing register (modal voice).
Find the highest note your student can sing in his normal voice. Play the note while your student sings a vowel. He should hold the note out and concentrate on bringing it up in pitch. Since this is already the highest modal note he can sing, his vocal cords will stretch and tighten. The falsetto voice will kick in. He should keep a conscious effort to avoid chest vibration, which signifies he's losing the falsetto tone. Once he reaches falsetto, he should sing the same vowel sounds in falsetto voice, keeping his hand on his chest as he tries to sing a scale upward.
Practice scale exercises in falsetto voice. This will help build power in this range. Due to the limited dynamics of falsetto by nature, your student will never be able to sing as powerfully in falsetto as he can in his normal singing range. It is possible to increase stamina, however. In addition to scales, use the classic yodel to help your student gain control over his falsetto notes. Limited power and lack of control are the two things that characterize poor falsetto voice.