  The MEG experiment at the Paul Scherrer Institut (PSI) represents the state
of the art in the search for the charged Lepton Flavour Violating (cLFV) $\mu^+
\rightarrow e^+ \gamma$ decay. With the phase 1, MEG set the new world best
upper limit on the $\mbox{BR}(\mu^+ \rightarrow e^+ \gamma) < 4.2 \times
10^{-13}$ (90% C.L.). With the phase 2, MEG II, the experiment aims at reaching
a sensitivity enhancement of about one order of magnitude compared to the
previous MEG result. The new Cylindrical Drift CHamber (CDCH) is a key detector
for MEG II. CDCH is a low-mass single volume detector with high granularity: 9
layers of 192 drift cells, few mm wide, defined by $\sim 12000$ wires in a
stereo configuration for longitudinal hit localization. The filling gas mixture
is Helium:Isobutane (90:10). The total radiation length is $1.5 \times 10^{-3}$
$\mbox{X}_0$, thus minimizing the Multiple Coulomb Scattering (MCS)
contribution and allowing for a single-hit resolution $< 120$ $\mu$m and an
angular and momentum resolutions of 6 mrad and 90 keV/c respectively. This
article presents the CDCH commissioning activities at PSI after the wiring
phase at INFN Lecce and the assembly phase at INFN Pisa. The endcaps
preparation, HV tests and conditioning of the chamber are described, aiming at
reaching the final stable working point. The integration into the MEG II
experimental apparatus is described, in view of the first data taking with
cosmic rays and $\mu^+$ beam during the 2018 and 2019 engineering runs. The
first gas gain results are also shown. A full engineering run with all the
upgraded detectors and the complete DAQ electronics is expected to start in
2020, followed by three years of physics data taking.
