Step 1: Repeater Bridge
Step 2: Firmware
Following the DD-WRT steps set the router up as a repeater/bridge. (this step is different for every router so checking the compatibility and downloading the most up to date and correct firmware is very Important.)
I did the same thing to make a wireless repeater for an ArDrone in this video.
Step 3: Connection to Router
Step 4: Tuning
To aim the yagi antenna Start with line of sight just look down the antenna like a gun and get it close. Then find a program that shows broad band signal strength (I used a laptop and a the long cable before I ran it through the house) Fine tune the antenna to the highest signal strength and tighten the bolts.