I've decided I need to start over. Learn kendo from the beginning again.
First, kendo is all about cutting with your feet. I know that if you teach this in a beginner class you probably won't have many students left. But still, this is important. If you don't get the feet right, then it doesn't matter how beautiful the arms and wrists are, it won't matter. Of course, as a beginner all you see is the shinai, and how it is used to wack someone over the head... but now I'm beginning to regret not concentrating enough on my feet.
Today I fully noticed how weak my left leg is. Left hand, left arm, all that, it doesn't matter if the left foot isn't there. I'm going to start training right now.
So my exercises are stretches, and straight leg lifts while lying on my stomach, and ankle raises while standing on something (like a small ledge). This will train the two muscle groups indicated in red, above.
The inset photo is of me (I had to flip it around, so I'm moving with the opposite foot), as you can see my muscles are weak and I'm leaning forward. Right, lets get to work and fix it.
Edit: found another exercise. Stand on right foot and kick left leg behind you as far as you can go (keeping your back and legs straight) and also contract your calf muscle so that the ankle kicks out (as per the picture above). If you can do this well then all you need to do is change feet (left toes ground, lift right foot) and you'll be able to do stamping.