A costly and time-consuming process. It takes up to almost one hundred days to prepare the dye, called sukumo in Japanese, made from dried polygonum leaves. The dye is then mixed with lye and lime and fermented.
The dyeing is usually done by hand, by dipping the garment in and out of the dye pulp. The more dips, the deeper the shade of indigo. Natural indigo, unlike synthetic form, is colorfast and its will not run when washed.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary