I'm going to guess that the image is supposed to be printed on a standard-sized piece of printer paper (before laminating). Going to guess "US Letter" 8.5x10" (215.9x279.4mm), but to be certain check the aspect ratio, if it's close to 1.414 then it's probably supposed to be A4 (210x297mm). And make sure that the square prints as a square :)

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.