How To Convert a Wireless Router into an Access Point
Linksys WRT54G
Step 1: Connect a computer that is set to obtain its IP address information automatically to a LAN port on the wireless router that you want to convert to an AP. If you don't know how to do this, Figure 1 shows the applicable Windows XP screens. (Get to the Network Connections window by Start > Settings > Network Connections.)
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Figure 1: Checking for "Obtain an IP address automatically"

Log into the admin page of the wireless router that you want to convert to an access point. From here on, I'll call this the "AP".
Step 2: For simple, one segment LANs, there must be only one DHCP server. Your LAN's router has a DHCP server and you don't want the two to conflict. So turn off the DHCP server on the AP. In Figure 2, you can see that the DHCP server in the WRT54G AP has been disabled.
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Figure 2: Shut off the DHCP server; change the IP

Step 3: Find your LAN Router's DHCP server range. Figure 3 shows the Basic Network Settings page of the D-Link DGL-4300 that is my LAN's router. This is where the DHCP server controls happen to be. Different routers might have the DHCP server controls on a separate page, so you may have to poke around a little.
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Figure 3: LAN router settings

The 4300 normally has its Router IP Address set to 192.168.0.1 by default. But you can see in Figure 3, I changed it to 10.168.3.254. This shifted the DHCP server to the 10.168.3.X subnet instead of the 192.168.0.X. I also could have used 10.168.3.1 or any address as long as it wasn't between 10.168.3.100 and 10.168.3.249, which is the DHCP server's range.
The bottom line is that the my LAN router's DHCP server range is 10.168.3.100 to 10.168.3.249.
Step 4: Change the address of the AP to an unused LAN IP address. This will ensure that you can reach the admin pages of the AP to change