1 – Dowload firmware and upload over tftp
http://wiki.openwrt.org/toh/asus/rt-n13u
http://luizluca.blogspot.com.br/2012/01/openwrt-turbine-seu-roteador-instalando.html
2 – install luci
http://wiki.openwrt.org/doc/howto/luci.essentials
3 – Install kernel drivers for usb thetering
https://www.google.com.br/search?client=ubuntu&channel=fs&q=cdc_ether+param&ie=utf-8&oe=utf-8&gfe_rd=cr&ei=nuKAVZOYAdCJhASt04CwBQ#channel=fs&q=cdc+ether+openwrt
https://downloads.openwrt.org/snapshots/trunk/ramips/rt305x/
https://downloads.openwrt.org/snapshots/trunk/ramips/rt305x/packages/base/
http://wiki.openwrt.org/doc/howto/usb.tethering