Practice major and minor scales daily using a metronome to gradually increase your speed. Play the scales in a 16th-note pattern ascending and descending. Start at 60 beats per minute while learning the scale, and increase by two beats per minute each day to a maximum of 144 beats per minute.
Memorize chord types and learn how to play them on the piano. Start with major, minor, augmented and diminished triads. Once you are comfortable with basic triads, add seventh chords, ninths and 13th chords.
Learn how to recognize jazz chord symbols immediately. This will require a great deal of practice and the use of a music theory text to learn the theory necessary to analyze jazz piano lead sheets.
Improvise by using the chord symbols above the music to play notes that fit with the indicated chords. You can add non-chord tones in between main beats, but the main beat of each chord, should be a chord tone when possible. For instance, if C major appears above a lead sheet, you can choose from the notes, C, E and G for the main beats. Anything that occurs off the beat does not require the use of a chord tone, so you could play a D or an F between the beats if you like.
Study the terms used in jazz music theory. There are hundreds of terms that can be learned from studying privately and reading a book on jazz piano playing.
Join a jazz ensemble through a local community organization or a local college. The best way to learn to improvise, once you have learned theory, involves jumping in and playing.