Generally, what HillRider mentioned is the general way of doing it. However, from personal experience (I have used at least 4 different brands or makes of expanders), it isn't always that simple. Most of them are similar but different enough to create problems and they usually don't come instructions. I have used the same one you linked. It has a "top cap" that goes over an "inner cap". There is a bolt that goes into another bolt. You tighten the inner cap before tightening the top cap. If you overtighten the top cap, the bolts can get stuck and you can't get the inner one out.
I found this expander to only work well on some forks. If you want a better solution, go with the FSA expander. That is the best one I've come across.