I have computer A: IP router

computer B: accessible straight through the Internet, IP gateway (switched, has its own IP, eth0)
Computer B also has a wifi network and it's (wlan0)

By doing ssh from computer A and using -w 0:0 I have linked A into B's wifi network, using:

A> ifconfig tun0 up pointopoint
B> ifconfig tun0 up pointopoint

I also did:
A> route add -net netmask gw
so I may access B's network from A. Also:
B> arp -sD wlan0 pub
to let them talk to A.

A is now effectively on B's network.

However, I'm now stuck. I want to use tun0 for the Internet, and as such I need to connect through the tunnel, into B's ethernet device as well as it's wireless.

How can I do that? I'm assuming you have to use iptables and routing?

I hope I made myself clear as possible.