What’s the basic procedure to draw a QCD phase diagram?

It is rather a complicated question and still an area of active research. The most reliable and direct way to plot the phase diagram with the axes $mu, B, T$, where $mu$ is baryon potential, $B$ is magnetic field is by means of Lattice QCD.

However, there is a important limitation - that the theory with baryon chemical potential suffers from notorious sign problem. This makes the direct simulation not feasible and results are obtained with imaginary chemical potential $i mu$, and then analytically continued to the real axis. There also exist different approaches for handling the sign problem - reweighting, complex Langevin, but none of the gives fully satisfactory results.

As a not very recent but of a good quality review I recommend https://arxiv.org/abs/hep-lat/0701002v1.