Get DD-WRT firmware from http://www.dd-wrt.com/

Install DD-WRT into router.

Setup the DD-WRT as a wireless access point (wap)

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

Then setup up multiple ssid

http://www.dd-wrt.com/wiki/index.php/Multiple_WLANs

Need to add the following in dnsmasq in the services tab.

without the comments.

# Enables DHCP on br1
interface=br1
# Set the default gateway for br1 clients
dhcp-option=br1,3,192.168.2.1
# Set the DHCP range and default lease time of 24 hours for br1 clients
dhcp-range=br1,192.168.2.100,192.168.2.150,255.255.255.0,24h

I used the following in the firewall setting.

iptables -I FORWARD -i br1 -m state --state NEW -j ACCEPT
iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
iptables -I FORWARD -i br1 -d `nvram get lan_ipaddr`/`nvram get lan_netmask` -m state --state NEW -j DROP
iptables -t nat -I POSTROUTING -o br0 -j SNAT --to `nvram get lan_ipaddr`

This was done using the firmware version,
DD-WRT v24-sp2 (03/25/13) mini
(SVN revision 21061)