Start by checking if the router supports DD-WRT custom firmware. Download and install the firmware according to the instructions for the router model.
Connect an ethernet cable from your computer to a LAN port on the router.
Enter the local address of your router (192.168.1.1 D-Link, 192.168.0.1 Linksys, 192.168.2.1 Belkin) into your browser and log in using the default credentials. The default credentials are usually root for the username and admin for the password.
Navigate to the Wireless tab and select Wireless Security. Select your current wireless security settings and enter your wireless encryption key that you use to access your wireless network. Click Apply Settings. It will be helpful to have a laptop on hand to double check the settings in the primary Wireless router.
Navigate to the Wireless tab and select Basic Settings. Select Client Bridge from the Wireless Mode drop down. Enter the name of your wireless network into the Wireless Network Name (SSID) box. Note: This is case-sensitive! Click Apply Settings.
Select the Setup tab, select Advanced Routing. Change the Operating Mode to Router and click Apply Settings.
Navigate to the Security tab and select Firewall. Disable the SPI Firewall by checking the Disable button and clicking Apply Settings at the bottom of the page.
Navigate to the Setup tab and select Basic Settings. Here is where you give the wireless bridge router its new local IP address other than the default. If the main router has an IP address of 192.168.1.1, set the wireless bridge router to have an IP address of 192.168.1.9. Basically make this anything other than the main router IP address or anything that falls within your DHCP pool.
Enter the Gateway and Subnet Mask of your wireless network next. You can find this information under Default Gateway and Subnet Mask by entering ipconfig in the command prompt (run: cmd) on a computer already connected to your wireless network. Click Apply Settings when the settings have been entered.
At this point you will lose the connection to the web interface of the wireless bridge router since the IP address has now changed. If you need to reconnect to the wireless bridge, simply enter the new IP address you selected above into your web browser.
Optional: Click the Setup tab and select Basic Setup. Under WAN Port, check the box to Assign WAN Port to Switch. This will add the WAN port to the 4 port switch, giving you 5 ports to share the wireless connection.
Run ipconfig /release and ipconfig /renew from a command prompt to reset the computers IP address.