Steve Kaufman says he can't emphasize enough the importance of developing a large repertoire of tunes. Fiddle tunes give us melodies that eventually allow us to begin to improvise. Sometimes without even knowing it. The more tunes you know, the more melodies and intersecting melody lines you know. This allows you to eventually cut and paste portions of the melody line into another song. In the beginning you will probably unknowingly replace and ending run. These are the last two measures of the section of a tune. Many of them in the same key are interchangeable, unless they are specific to the melody of the song. After a while you will cut out measures in the middle of the song and then you will be able to take the snips of melodies and play them in another song in a different key thus beginning to improvise. All of this is possible because you built up your repertoire of tunes.