Hello,
Tunel should be created between ADSL modem and Linksys PAP2T (marked with red).
Both devices are connected to free ethernet port.
How to connect two ethernet ports over wireless?
Look at the various tunneling protocols in the wiki, complete with diagrams and example code. There are several to choose from: PPTP, L2TP, EOIP, IPSEC ,OVPN, SSTP, etc…
Study them all and then you can choose the best one for your needs.