Among the many accounts of how art forgers go about their.... craft, the story of John Myatt and John Drewe is particularly worth reading.
(Myatt, by the way, also appears in Who the #$&% Is Jackson Pollock? as an expert commenting on the difficulty of convincingly forging a Jackson Pollock.)
The genius behind this scam was not so much the quality of the forged works painted by Myatt, but the way in which Drewe 'helped' the art world authenticate them. As the article teaser for Peter Landesman's piece on the subject reads:
There's now also a book on the affair.