Step 1: Is it dry? Yes ==> BW (desert) or BS (steppe)
Step 2: Every month > 18°C ==> Af, Aw, As
Step 3: Every month < 10°C ==> ET (tundra), EF (frost), EH (mountains)
Step 4: C or D-climate?
C-climate: winter > -3°C ==> Cf, Cw, Cs
D-climate: winter < -3°C ==> Df, Dw, Ds