Players such as Bud Powell, Thelonious Monk, Charlie Parker, etc., invented the jazz genre of bebop. Today, I am told to be good at playing bebop you must listen to the best jazz players and transcribe what they played in order to assimilate the language. But since players such as Powell and Monk had no bebop predecessors to transcribe, what were some things that they practiced to invent this genre of jazz?
A quick note at the beginning--it's important to remember that the solos a bebop musician plays are not exclusively the result of practiced notes. They are a result of ideas one hears, intuitions one has developed, and technique one has established.
Influence of Swing
Many bebop players were heavily influenced by swing. Charlie Parker memorized Lester Young solos and studied Coleman Hawkins extensively. Many of the melodic and even rhythmic elements that define bebop can be heard in smaller doses in swing. Swing doesn't explain everything we hear in bebop, but much of bebop is an adaptation of ideas applied less complexly in swing. So the first answer to your question is: many of the musicians who created bebop spent a lot of time practicing and playing swing.
Open Exchange of Ideas
Another important aspect to bebop's evolution is that it occurred collectively--in groups and in shared spaces. The masters of bebop created the art form literally while standing on the stage (e.g., at Minton's) experimenting in small combos together. By taking turns improvising, these bebop greats were practicing, experimenting, and influencing each other all at the same time. Bebop is an outgrowth of new musical ideas, and by playing together in small groups, they helped to train each others' ears and musical intuitions. Features like high tempos and trading 8's, etc. served as a hotbed for progress. If someone happened to play an especially interesting idea, the other musicians would try it out as well when it came their turn to improvise.
The same experimentation that occurred on the stage was put to practice through composition: a major form of practice for bebop players was to compose bebop heads. Many players then practiced those songs at different tempos and in all 12 keys. These songs often took the form of contrafacts, which became a cornerstone of bebop. These bebop melodies utilized the exact same techniques being put into practice when improvising. So by writing bebop heads--and many were written--one was in effect writing out (and hence, practicing) their improvisation.
Normal Practice Routines
Many bebop players were obsessed with practicing, and they practiced anything and everything they could think of or find. To use Charlie Parker again, he said in an interview with Paul Desmond that he spent a few years practicing 11-14 hours a day. He was known to practice the Klose exercises, to practice in all 12 keys, to practice arpeggios and scales, and to practice building melody from higher chord extensions. He often practiced small bits of songs over and over (2-3 measures at a time), experimenting with different melodic connections and moving licks to different parts of the beat. He also said that he studied Stravinsky in his later life. So a lot of the normal things we expect someone to practice were present for the bebop musicians. This contributed to general technique, which allowed them to express their unique ideas successfully.
Innovation and Genius
Bebop was a highly experimental form of music, and it's undeniable that there was some true innovation occurring. The era was characterized by profound experimentation and genius. Monk is a great example--only so much of what comes from Monk's mind can be attributed to influence; the rest is pure creativity.
Ultimately, bebop musicians practiced just about everything. They practiced swing, they practiced classical music, and they practiced bebop itself.
As some final tidbits, you can hear Parker practicing here, and you can hear a very early 1939 version of Body and Soul played in an early bebop style by a master of swing. It is one of the first recordings created with a bebop-esque feel.
Just one data point, about Charlie Parker. I don't have the book with me to get all the details, but I remember reading the following in Miles Davis' Autobiography:
A few days before Charlie Parker died, some other musician happened to see him alone in Central Park. Parker looked tired, they had a short talk, and when they parted Parker donated to this other guy a bundle of heavily used scores. It was classical music. (I don't remember exactly which pieces, I think they were string quartets. I think it was mentioned quite specifically in the book). A few days later Parker died, and many in his social circle came to know about the story of his classical scores.
Miles Davis goes on to say that he as well as many others had been wondering whether Parker had ever studied or practiced classical music. Many suspected he had, based on certain details of his phrasing and style, but he himself never admitted it. The above story however seemed to positively confirm that he did indeed.
There is a story that when Charlie Parker was young he got embarrassed on stage trying to improvise. Realizing he didn't know anything about keys or chords he woodshedded on Cherokee by playing the changes in all 12 keys.
I first read it in a booklet from Jamey Aebersold. The original source is an interview between Parker and Leonard Feather called "Rappin with Bird." I've not actually heard the recording. The text is:
CHARLIE PARKER: “I knew how to play two tunes in a certain key, the key of D for saxophone (alto sax), F concert. I learned how to play the first eight bars of Lazy River and I knew the complete tune to Honeysuckle Rose. I never stopped to think about there being other keys or nothin’ like that. So I took my horn out to this joint where a bunch of fellows I’d seen around were, and the first thing they started playing was Body and Soul, long-meter, you know. So, I go to playing my Honeysuckle Rose and there ain’t no form of conglomeration, you know so … they laughed me off the bandstand … they laughed at me so hard ….. I was about 16 or 17 at the time. I never thought about there being any more keys, you know.”
I've always liked that story, because it's so specific and right from the man himself.
I couldn't find a published copy online, but I was able to find an interview between Parker and Paul Desmond on Youtube that sheds some more light on the subject.
Parker specifically mentions "studying" the horn 11-15 hours a day for about 3-4 years and "...it was done with books."
There really is a second part to your question: how did they develop be-bop. Here are some good links to sources...
...where Parker talks about playing the upper chord extensions to get the sound he had been hearing in his head, and about making a new tune over the Cherokee changes to avoid copyright royalties. Parker mentions the composer Hindemith a few times which is a great insight. Also, that linked Google book recounts the story about getting laughed off stage in his youth.
I mention these points about Charlie Parker only because I've seen them many times in many places, and they seem well documented. I wish I new more about Thelonious Monk. I haven't tried researching him, but I imagine there are good sources available.