Two years ago, I did this to two of my Buddleas, just find the straigtest longest cane that is as close to the middle as possible. Get a long bamboo pole (I used bamboo and rebar for strength, Hold your breath and cut back all other canes to the base, then push the stake(s) as close to the plant as you can, tie the cane you selected to the bamboo after removing all side leaves. As this grows, continue pinching all side shoots until it gets the height you want (one of mine is almost 6 feet tall, the other is about 4' tall, then you pinch out the center growth bud. It will then send out all the side shoots. I then planted ivy to grow up the stake. I was told that buddlea canes die after about 3 years, so this year I let a new cane growfrom the bottom, normally I would pinch them all off, and I am tying it up alongside the original one, pinching side shoots as described above. I can then cut down the other one if it looks like it isn't producing, but it looks great this year again. Sort of like a tall umbrella. It was quite easy. Every year in late winter when I am cutting back my buddleas, I cut this one back at the top, cutting all the green growth to the hard wood that has formed 6 feet up. Pretend that the top is at ground level and do what you normally do about pruning.