At which stage is lishma required?
It is proper to declare the intention of lishma when tanning the skins for stam. Bedieved, an unspoken intention is also valid. In the tanning process, the hides are first soaked in water and then in lime. The lishma declaration should be made after the water and before the lime. The poskim discuss whether a declaration which preceded the water is valid if the necessary declaration was not made at the time of placing the hides into the lime.
(סעיף ח וס"ק כד-כה; ביאורים ומוספים דרשו, 43)