In general terms...
You'll want this in Access Point (AP) mode.
First, determine where you'll put it (in the weak coverage area)
Then, how the AP will connect to the router:
cat5 cable, AP to router via attic, under-floor, etc. or...
a pair of HomePlug/HPNA devices to use power wiring instead of cat5, or...
a pair of MoCA devices to use existing TV coax cable outlets
Once that's decided, you'll first connect the router/AP to your PC direct or via the router's LAN port. You'll follow the instruction manual:
setup Wireless in the AP: SSID (I recommend it differ from the router's SSID), encryption mode and passkey. AP mode. Give the AP an address in your LAN's IP range but not in conflict with the router's DHCP assignment range of IP addresses. You'll use this IP address for the AP just to admin it.
Then deploy.