The most effective way to learn how to play jazz is to start by learning how to do what a bass player does: play the root notes of all the chords in the right spots.
The root notes are like fence posts that appear on a continuum or time line. They tell you the order and sequence of different tones that make up the shape and structure of a song. For example, for “Happy Birthday” in the key of C, the bass player plays C for 3 beats, then G for 3 beats, then G for 3 beats, then C for 3 beats—and so on until the soundscape of “Happy Birthday” is established. The bass player is creating a sonically fenced-off area that says, “only a certain set of notes fit here sweetly.” (Any notes that don’t belong to that family of notes are considered dissonant or salty. Dissonance isn’t a bad thing. It’s more like flavour, context, or controversy—but that’s another topic altogether.)
I think of the melody notes to “Happy Birthday” as sweet little cherubs that fit neatly into the bundle of chords staked out by the bass instrument. The bass notes are like the walls of a little playpen that holds all those giggling little cherubim together. There are other notes that fit into that playpen nicely as well, and when you add them, they sound like bows and ribbons and little booties. (By the way, heavy metal songs have more jagged shapes and get a different treatment altogether.)
The important thing to know in the beginning is how to make the playpen that holds all this loveliness together. Each instrument, from flute to vocalist, saxophone to trumpet, must learn how to do this in steady time. Once budding musicians can do this and know exactly what fits in the structure, they can improvise and create their own scenarios and make their own decisions about how it will be embellished. It’s sort of like playing dolls with musical tones. (Naturally there’s always that individual who has a penchant for dissonance and wants to place a voodoo doll or a zombie in there amongst the cherubim. Those are the Miles Davis types. Again, I can say more about that on another day.)
It’s good to practice this skill on standard song forms like the blues or “Autumn Leaves” or rhythm changes. Then step out of your comfort zone and try out your ideas at Live at Massey Hall, Vincent Massey’s terrific jam session for players still learning how to improvise. Check out what happens at the Cool Wednesday Night Hang over at the Orbit Room. For an introductory course in improv, you can check out the U of M Jazz Camp each August, or audit Professor Quincy Davis’ Rhythm Analysis course in the Jazz Studies program at the U of Manitoba.