I also had issues with using the planar command to create a flat plane at the center of an object. For some reason my planes were massive in comparison to the ones in the video. I could not avaerage them out to find the central point. I was interested in learning about notching from creating a circle on a plane between two surfaces. This would be extremely helpful for manufacturing purposes.
offsetting the angle
Creating different parts to the circle
Creating a notch for the material to fit into. Obviously the notch has been exaggerated here
Ofsetting the line
Conturing a surface
By creating a Brep component out of subtracting different extruded shapes from my square (the Boolean subtract command in Rhino) I was able to use what I learnt in the driftwood 8
The contouring was placed against an offsetted surface to create contour lines.