Today, a colleague and I were looking at a new book on improvisation by an author who is well known but I won't mention here. After looking it over, we both agreed that a better title might be "The Tedious Study and Science of Improvisation". The book was thick and so overloaded with information that if someone was trying to understand how to approach improvisation based on this, they might be so seriously confused as to give up trying to play completely. Over the years I've been asked how to approach improvisation. I always try to keep it simple, because in reality it is. This article is really addressed at those who would like to start improvising, maybe attend a few jam sessions but aren't quite sure how to do it and what to do. Let's take the mystery out of it. Improvisation is not some esoteric or mystic thing that only the few initiates can do. Anyone can improvise. This article will be far smaller than that book.
To start with, the basis for becoming a good improviser is to study and practice your scales and chord patterns. However, the most important aspect of being a good improviser is to be a good listener. When you study your scales and chord patterns combined with listening to them, you'll start to hear outlines of melodies that you know and then start hearing melodies that may be original. Many composers past and present have written great compositions that came out of their improvisations. Study and play the scales and chord patterns in all keys, and then as you listen to music you'll start to recognize those scales and patterns that are used by other players. Then, as you continue to listen and play, you'll even start to embellish a song you know with your own ideas. You'll find your own approaches to playing various musical phrases. The most important thing is to approach this in the spirit of self-discovery and fun.
I recommend the Jamey Aebersold and Hal Leonard Play-A-Long books to practice improvisation in the comfort and privacy of your own home or room. Jamey Aebersold Volume 1 is especially useful in demonstrating to the novice a very practical, easy and fun approach to jazz improvisation, employing some simple but effective exercises for hearing and improvising. You play along with the rhythm sections on the CD's that accompany the books, and in a very short time you should understand the really simple approach to what it takes to come up with your own lines and phrases.
I also recommend Jerry Bergonzi's "Inside Jazz Improvisation" volume 1, because the basic premise is that we take a chord pattern, and rather than look at the notes as C, D, A, etc., we take the number of notes in that chord and from the root, count each note as 1,2,3,4, and that every chord has 24 permutations, meaning when we practice these patterns, we will mix up the order we play these chords 24 times. This enables us to not only hear each chord pattern a little differently, but to give our ears and fingers a different way of playing these chord patterns for different tunes. So for example if you play a C major chord, the notes are C, E, G and B. Now they become 1,2,3 and 4. You play it first in that order, then go 4,3,2,1, then 2,4,3,1, and so on until you've played all 24 permutations of that chord. This trains your fingers to mix up the way you play any pattern so as to not be monotonous when soloing and improvising.
When you have practiced enough to gain at least a basic understanding of improvisation, then go out and look for jam sessions and open mics. Don't be afraid to get out there and hone your chops. Don't worry about whether or not you're going to play great. Of course, in the beginning, I don't recommend going to jam sessions where the real heavy cats play, because unless you're at their level, they will be positively brutal to anyone that can't cut it. There are plenty of jam sessions and open mics where interested amateurs play and don't try to cut each other to pieces, just jam for the fun of it. Most of the tunes at these sessions are simple blues or pop tunes. Just go out there and have fun.
Once you have gained a good knowledge of scales and chords and have practiced them extensively and gained proficiency on them, then basically forget about them and just open your ears when it's time for you to improvise. If you've studied and practiced your scales and chords and pay attention to what's going on around you, then you'll automatically begin playing whatever comes into your head without worrying or wondering about which note, scale or chord to play. It may not come out perfectly every time, but so what. The main thing is that you continue to explore melodic and tonal possibilities and above all, have fun.