As documented on: http://www.dd-wrt.com/wiki/index.php/Wireless_Access_Point

Here's how to create a Wireless Access Point using dd-wrt v24. Please pay special attention to the Review section of this article, especially if you are using an older version.

1. Hard reset or 30/30/30 the router to dd-wrt default settings
2. Connect to the router @ http://192.168.1.1
   • Note: If this router is wired to another router, there may be conflicts (both routers could have the same IP address). For the time being, disconnect this router from the main one.
3. Open the Setup -> Basic Setuptab
   • WAN Connection Type : Disabled
   • Local IP Address: 192.168.1.2 (i.e. different from primary router and out of DHCP pool)
   • Subnet Mask: 255.255.255.0 (i.e. same as primary router)
   • DHCP Server: Disable (also uncheck DNSmasq options)
   • (Recommended) Gateway/Local DNS: IP address of primary router (many things will fail without this)
   • (Optional) Assign WAN Port to Switch (visible only with WAN Connection Type set to disabled): Enable this if you want to use WAN port as a switch port
   • (Optional) NTP Client: Enable/Disable (if Enabled, specify Gateway/Local DNS above)
   • Save
4. Open the Setup -> Advanced Routingtab
   • (Optional) Change operating mode to: Router
   • Save
5. Open the Wireless -> Basic Settingstab
   • Wireless Network Name (SSID): YourNetworkNameHere
   • (Optional) Sensitivity Range: The max distance (in meters) to clients x2
   • Save
6. Open the Wireless -> Wireless Securitytab
   • Note: Security is optional, but recommended! Clients must support whatever mode you select here.
   • (Recommended) Security Mode: WPA2
   • (Recommended) WPA Algorithm: AES
   • (Recommended) WPA Shared Key: >8 characters
   • Save
7. Open the Services -> Servicestab
   • (Optional) DNSMasq: Disable (enable if you use additional DNSMasq settings)
   • (Optional) ttraff Daemon: Disable
   • Save
8. Open the Security -> Firewalltab
   • Uncheck all boxes except Filter Multicast
   • Save
   • Disable SPI firewall
   • Save
9. Open the Administration -> Managementtab
   • (Recommended) Info Site Password Protection: Enable
   • (Recommended) Routing: Disabled (enable if you need to route between interfaces)
   • Apply Settings and connect Ethernet cable to main router via LAN-to-LAN uplink*

• Notes:
  1. To connect the WAP to the main router, you can probably use either a patch cable, straight-thru, or a crossover cable. Most DD-WRT capable devices can do auto-sensing so the cable type doesn't usually matter.
  2. You can connect the WAP to the main router via LAN-to-WAN so long as you have assigned the WAN port to switch (see step 3).