First thing to do is to loosen the big screw on the bottom clamp so that the clamp is really loose. Then you slip the channel over the lip on the back of the pedalboard (the channel is lined with red felt) and slide the clamp underneath the pedalboard. Move the shoe into place and tighten up the screw to secure the shoe in its place.
The key is to really loosen that bottom clamp. That eliminates most of your problems.