"The author discusses the detailed and painstaking method used by the painter at each step of the creation of the painting to obtain sfumato effect."
I thought perhaps this was worthy of its own thread...
"Reconstruction of the painting was preformed in five principal stages: preparation of the panel, preparatory sketch of the composition, underpainting, glazing technique and scumbling technique. All used materials were either identical or at least equivalent to the ones Leonardo used.
Where traditional pigments were hard to obtain, due to their toxicity and/or price on the market, contemporary substitutions were applied."
... and here's the link to the detailed methods, a wonderful read: