Considering the low-temperature ($T$) and high-baryon-number-density ($n_B$) region of the QCD phase diagram, we present a model for the first-order phase transition between the quark-gluon plasma (QGP) and the recently proposed colour superconducting phase. We study nucleation of a droplet of the superconducting phase within the metastable QGP gas. Numerical results for the activation energy, radius and other physical parameters of the droplets, at various temperatures, densities and gap parameters, are given. We have estimated the latent heat of the phase transition. In the $T-n_B$ plane, we are able to demarcate the region of the superconducting phase.