For a singer, improvisation is a combination of things. Having a voice agile enough to move around, a developed ear for harmony and rhythm, a professional musician's understanding of what the experience of making music should be, and a spiritual or philosophical point of view of the musical experience and the part you directly take in it.

Firstly, why do you want to improvise? Hopefully, it will be somehow that you want to meet the challenge of saying something meaningful (beautiful, exciting) in a precise language within a precise form. These are just my words of description, but I am describing a fact; a happening; an experience that is real -- that any can and many have experienced, and perhaps put into other ideas and words. But there is a road to travel down, and it can be an act of exquisite balance and grace. Or it can be hell.

The very first thing you have to be is yourself. The danger and excitement of this is very real - here is a space in the music -- what do I want to do in it? Then how the inner voices start! "What if I do something stupid, ugly, wrong..." "I can't do this!" "He thinks I'm terrible!" "What do I do here?" "Where am I?!" STOP! Enjoy the moment, the dimensions...observe, and then go forward into it. Have PATIENCE. And by the way, the moment also includes everything there - the room, audience, your emotions, the band, etc... don't resist the environment, its part of the experience.

Of course you have to know as much as possible about the musical form you are observing. The sections of the song: How long they are, what chord progressions make them up (even if it's only by recognizable sounds),what happens after each one. Then for your approach.

1. Find the groove (up tempo, medium or ballad) and lock into it.
2. The beginning improviser will, no doubt, need to hear each chord played first for security to know what notes can be sung. A little more experienced improviser should look down the road - always know what chord comes next ... this will lead and form your melodic phrases.
3. Hear a beginning note inside your head that preferably is part of the makeup of the chord that's being played and the mood of the color of the song.
4. Realize you're not just going for one phrase. Have the idea that you're creating a storyline from the beginning of the song through to the end. It just happens to be made up of melodic phrases.
5. Start with a simple phrase...compose it like part of a song or make it up with parts of scales. You can (and should!) sing a phrase of the real melody from time to time. Use syllables that sound like a sax...doo bee dah bup, ba ba doo n doo bop...
6. Repeat the phrase... change it slightly every time. (In classical music they call this developing the motif.) (Again,for beginners, its good to fit the notes in with the chord you are now in, or hearing.)
7. Change the rhythm of your phrase - hold notes out longer, or put in a rest somewhere or start it on a different beat.
8. Use part of a chromatic scale in the phrase.
9. Try carrying the phrase over into the next measure without breaking at the bar line. You are connecting chords this way and it makes for a fluid action. There are common tones between chords that make this action easier.
10. What you sing is who you are. It's a conversation you're having. You breathe, you pause, you get excited, you laugh, you're in love, you're sad, you're sexy.
11. Listen to, interact with, respond to the rhythm section! It's just as if you are in an intimate relationship with someone - through the music, you allow them to touch a part of you that sets off a spark. (Again - the danger and excitement of being yourself!)
12. Use different rhythms, qualities of tone (your instrument is unique! Find your special tone!), high and low range, volumes. Explore 'tension and release'. These 2 elements make interest. Any good piece of creation has a buildup of tension and a release. You can choose where you want the most tension, but generally, toward the end is a good place to end up with the most tension, and then a release. Tension can be made by: building volume, ascending lines, emphasis on no-chordal tones (also known as passing tones), wide intervals, speed of notes, etc. Release can be made by: decreasing volume, descending lines, emphasis on chordal tones, using silent spaces, etc.
13. Improvise with decision and intention...no matter how loud, soft or subtle the music is.

Listening to recordings is a huge part of learning. Listen to singers like Billy Holiday, Sarah Vaughn, Ella Fitzgerald, Carmen McCrae, Anita O'Day, Betty Carter, Chet Baker, Mark Murphy. All of these singers improvise the song, lyrics and all, the 2nd time through the song...like rewriting the song. This is just as challenging as scatting through with no words (or syllables). And singers like Carly Simon, Linda Rondstadt , Natalie Cole have done "straight" versions of Standards, that you can learn the songs from - and then listen to the others to compare. Look at the lead sheet while you listen. Listen also to instrumentalists - especially horn players like Chet Baker. Chet is a simple and beautiful player to learn from. If you are at all able to musically write down the recorded solos in music form, do it! This works great for putting info into your head!

Practice is for making the instrument able and agile to accomplish the varying scales, intervals, and rhythms. And to deposit into the mind the varying possibilities harmonically possible. Performance is for creating, emotion, and fun ... backed up by the silent partner - intelligence, which practice has supplied.

When you go over a song to practice, play the chords, simply, if need be, or use a pre-recorded tape. Sing scales, get familiar with the chords of the song. Try starting the scales 1. after the chord changes, 2. when the chord changes, and 3. before the chord changes. Scat really concentrating and listening, and then freely - with no concentration.

In the end, improv should be a communication and a reflection of where you are right now, in this moment. And that is why it is a constantly changing vehicle. You should never expect or try to sing the same solo each time you perform the song. Reflect what you know and feel in the moment. The more you don't struggle with that, the better the solo will be. Be interested in the music, don't be interested in yourself.

And please, take the responsibility of being a learned musician seriously. Singers improvising are not just 'howling at the moon' freely. They have studied and are aware of these things I've mentioned here. Musicians will either run when they see you coming or welcome you with open arms...which do you want for yourself?