Producing carbon nanotubes from graphite
Graphite does not contain CNTs, a simple extraction therefore is not possible.
Graphite seemingly can be converted to CNTs by a "simple" oxidative method published by DW Lee and JW Seo on arxiv.org in 2010.
The authors claim to have used commercially available graphite powder (45 µm, Aldrich) of high purity (>99.995%), but did not provide any information on the yields.
Pencils typically contain a mixture of graphite powder and various amounts of clay as a binder.