back to basic always.
You know how to do a single step (lamda/4) transition, since you know the lamda/4 impedance matching formula, Zt^2=Zo*ZL
So same thing for multi-step transition, we can still choose lamda/4 for each step, and make the matching from load to the source. If you are lazy to calculate it, build a model in ADS etc, and optimize to find right impedance for each step.
The impedance for each step is determined by your waveguide width and height.