Many students struggle with the leap from technique practice to actually improvising and creating their own melodies. I'll be the first to admit that it's not easy, but there are a few things you can do to practice improvising and make the transition from practice room to bandstand less intimidating.
The reason I love this Bird quote is twofold: 1. Bird practiced his butt off. He wasn't some alien being that God dropped on the planet who magically started playing bebop. In fact, he spent 10 years practicing and playing in swing bands before the notion of bebop was conceived. The lesson here is that everyone, including Bird, must practice and apply that practice before any real change or personal style emerges. 2. It's clear that Bird sees creating and improvising as a much different animal than shedding in the practice room. He doesn't go into great detail so what exactly does he mean by "forget all that and just wail?" Well, I'll attempt to break it down for you.
The first concept that most young improvisers fail to grasp is that you are improvising RHYTHMS in addition to MELODIES. In fact, rhythmic integrity and variation is the foundation of good improvising. In today's harmonic/melodic environment where anything goes (there are no wrong notes), I encourage students to focus on practicing improvising using a basic rhythm first. Always remember - It Don't Mean a Thing if it Ain't Got That Swing. If you aren't in the pocket and playing with solid rhythmic integrity, it simply does not matter how hip your melodies are. No one will hear them because they'll be thinking "man, that cat cannot swing."
First, place your metronome with your tuner on your music stand and turn them on (notice a pattern here?) Set your metronome to about 66 bpm. Then, take a basic jazz rhythm like quarter note followed by 4 eighth notes followed by a quarter note rest. That combination of notes and rests equals one full measure in 4/4 time.
The next step, as you grow comfortable with the rhythm, is embellishing that rhythm, being very careful to stay within the groove. Here's a potential variation on the triplet rhythm stated above.
This exercise does a few things to help bridge the gap between practicing scales and licks and actually improvising. Primarily, it gets you out of the mindset of notes and into rhythms, which are really crucial to interacting with a rhythm section and building a solo via motivic development. A motive is a short rhythmic and melodic phrase on which you can build your improvisation. When I think of Sonny Rollins, I think of a great master of motivic development. He will take one rhythmic phrase and build an entire solo out of that rhythm. It also makes you focus on swinging - always placing your rhythms in time. Remember that if you aren't swinging, it simply ain't happening.
Try this rhythmic practicing concept over a tune you are learning. Start with a tune with minimal key changes like a blues or a rhythm changes and see if you can shift the focus from "making the changes" to "making the rhythms". Constantly build on your motives and vary the rhythms within the basic rhythm. I think you'll find that a new door will open for you and you'll start to think about truly improvising and building on motives rather than trying to insert lick #142 into the turnaround before the top.
There are few who reached the level of proficiency and genius of Bird, but if you get inside this famous quote from him that started this lesson, you can start to unlock a couple of key concepts. First, you must practice - learn your horn! I equate that to tone and technique practice in my regime. Second, "forget all that and just wail". It's not quite that simple, but I believe what he meant was to use your instincts, play with conviction, never forsake the groove, and don't be afraid to take a few chances. Practicing your rhythms can help you do this.