Pour in petri dishes before it solidifies, using a pipette and trying to have the same volume in all dishes. We usually pour the plates on the bench, but to minimize the chance of contamination one can pour them in a microbiology hood.
These are our usual volumes:
For 100x20mm (diameter x height) petri dishes, we pour 18 mL.
For 60x15mm petri dishes, we pour 8 mL.
For 35x10mm petri dishes, we pour 2 mL.