The OPERA experiment, designed to perform the first observation of νμ→ντ oscillations in appearance mode through the detection of
the τ leptons produced in ντ charged current interactions, has
collected data from 2008 to 2012. In the present paper, the procedure developed
to detect τ particle decays, occurring over distances of the order of 1 mm
from the neutrino interaction point, is described in detail. The results of its
application to the search for charmed hadrons are then presented as a
validation of the methods for ντ appearance detection