This kind of hack was really popular when wifi was first being adopted. These reflectors work best when they are spaced according to the wavelength. Here is a template I remember seeing years ago: http://www.freeantennas.com/projects/template2/
t looks neat, but it doesn't tell you how far away you should place it or how big to make it. Am I misssing something?
Then you follow the assembly instructions, which are indeed a bit minimal; The six "bumps" (called "tabs" in the instruction) sticking out the "Windsurfer" part are probably supposed to stick into the six cut-lines of the rounded rectangle piece you cut out from the bottom of the sheet.
This will make the "Windsurfer" part a particular curved shape. I suppose this is the "reflector", so you probably should glue the aluminium foil only to this part.
Then there's the two crosses on the "Windsurfer" part, which you also cut out with a sharp knife, and seem to me to be just the right kind of holes to pierce the whole thing on a wifi-antenna. Get the picture?
Some kind of photo of the finished end-result would have been nice indeed :)
Disclaimer: I know almost nothing about radio signals or antennas etc, the above is just my interpretation of the instructions on the site.