The most touristic way is to start climbing from the Escalera Ancha, cross the Fray Toribio de Benavente square and continue the ascent to the hill from the Tláloc ring road. The ascent itself will show you the way, since cars can only reach a certain point of the hill.  The trail from the base of the Escalera Ancha to the top of the San Miguel hill is approximately 1.2km and 22 minutes of ascent. A local will know how to climb more quickly and by more direct routes, although perhaps more strenuous for a non-native.