Many people don't know how to do this properly.
Here's a noob friendly guide:
Here's how to do it:
1. First thing, plug in the power to the wireless router, but do not connect it to your network yet.
2. Attach one computer using a category 5 network cable to one of the wireless router's LAN ports. Make sure this computer is set to automatically receive a IP address from the wireless router's DHCP server.
3. Login to the wireless router using the computer that is connected to it. Now that we're in the router configuration, we can make the changes we need to turn this wireless router into a wireless access point.
4. You can leave the WAN configuration in the wireless router completely empty. The WAN part of the wireless router won't be used when it is being used as a wireless access point.
5. Set the LAN IP address of the wireless router so it is compatible with your existing network. For example, if your current network uses the 192.168.1.x network, then make sure the wireless router is on the same network. Different companies use different LAN numbers such as 192.168.0.x or 192.168.2.x. We need the wireless router to function on the existing network. Give it an IP address that is easy to remember. For example, if your current wired router is 192.168.0.1, then you can give the wireless router 192.168.0.2 which is only one number away from the wired router number (which you probably already know by heart).
6. Turn off the DHCP server on the wireless router. There can only be one DHCP server on your network, and it should be the DHCP server on your wired router, not the wireless router.
7. Implement the same wireless security settings. You want to keep your wireless network as safe as possible.
8. Now that your wireless router uses the same IP numbering scheme and has the DHCP server turned off, it is basically a wireless access point. We have to hook it up to our existing network. You do this by connecting a Category 5 cable between the LAN port of your existing router / network to the LAN port of your new wireless access point. Do not connect it to the WAN/Internet port of the wireless access point. Depending on your router / access point manufacturer, you may need to use a crossover cable instead of a standard straight through cable. On my Netgear WGT624 wireless router turned wireless access point, I use a straight through cable without any problems. You may need to experiment with this.
There you have it. You added wireless capabilities to your existing wired network by using a wireless router as a wireless access point.