First get a clean plastic 2 liter pop bottle.
The as you see by the photo.... get out your tin foil and glue stick and get ready for the next step of lining the inside face of the bottle with tin foil and making it as smooth as possible.
Upon a few peoples suggestions..... It has been noted that if you make the cut at the edge and not the center the signal will be much higher. I totally re-did the cap as they suggested and my signal went up from 76% to 90. OK GUYS POINT TAKEN !! So having said that....
Adjust accordingly to follow RIMAR2000's suggestion to move it at least an inch or so closer to the reflector. You were right and I was wrong.
Also if you get a chance, follow the link that RIMAR2000 has on his comment and you will see where he got his information, granted, you will have to translate it, but, if you look at the diagrams... It all makes perfect sense !
Up to 76% and running great!
Funny thing is to about this is the router is over 100 yards away in our office at the place I live!
They offer free WIFI in the clubhouse and office, but, I can use it from a long ways away now!
Have fun and I hope this helps with any signal issues you may encounter in your WIFI network.
P.S. the antenna that I was using before was a 7dbi antenna and now it is on steroids!