Start with a yawning feeling to relax the throat and let the layrnx drop (this is the bump in the neck that moves up and down when you swallow).  Then let out a siren type of sound ("wooo" or "weee") and feel it go up into the head cavities. Now let out a "wooo", like your cheering for someone.  Also try hooting like an owl.  Pay attention to the feeling of resonance in your head. This is your head voice.

Keep the feeling that you are just starting to yawn and say the word "dumb".  Place your fingers on your larynx and feel it descend as you do this.  Sing a scale (dumb..dumb..dumb..dumb..etc.).  As you ascend, let your voice cross over into the same resonance area you felt in the previous exercise. Keep your fingers on your larynx to make sure it doesn't pull up.  You should begin to feel the shift from chest voice into head, and back again.  Don't worry if your voice breaks or cracks at first.  With practice, this will smooth out and disappear.  You will always feel the shift in resonance, but ultimately the listener will never hear it.

 Now say the word "mum" in your normal speaking voice. As you sing a scale on this word, don't let it get louder or strained as you go higher.  Keep it very light if you must.  Once again, allow it to cross over into your head resonance area.  It will feel weak at first, especially if you are used to pulling your chest voice too high into your range.

Never go higher than is comfortable and always stop at the first sign of strain.