Each pipe stand comes in two parts: the top and the base. The top is layed flat for easy shipping. Make sure you have a top and a base before assembly.

Pull the strap along the pipe stand until it catches on the hook end of the strap.
While holding the strap down with one hand, fold the pipe stand downward until the tabs at the bottom fit into the slots on the base (make sure the strap is facing up when folding).
Insert the tabs into the slots on the top of the base, repeat for the other side.
(optional) Using a flathead screwdriver fold the ends of the tabs up to ensure a tight hold. Note: when pipe is rested on the stand it will hold the tabs in place.
Locate the pipe this side sticker and place whatever diameter pipe you are supporting in front of the strap on the sticker side.
Slowly bring the strap from the bottom and fold it over the top while holding the pipe at the correct height.
As you will notice there is another slot on the back side of the stand, slide the strap through both sides of the slot. Bend the strap upwards to secure strap completely.

You're done! No more assembly is required.