oh.. so you mean he just wants the typical basic setup to put a wireless router in between his computer and the existing beetel modem???
then in most simple terms, remove the lan cable from your computer, put it in the line in of your linksys router, take another lan cable, put it in one of the lan ports of your router and connect it to your computer or connect wirelessly if you have wifi card or laptop.
most probably, you wont need to change any ip settings.. .because most of the standalone modems have 0 in their third octet, while the routers have 1.
all you need to do is configure the PPPoE protocol in your linksys router with the username and password of your broadband account, and spoof the mac address of the computer which was used when you bought the connection.