How do I temporarily set up the Ubuntu laptop to "serve" the home router?

[rocksockdoc]

The "normal" configuration, before the rooftop antenna fell down, was:

• Wireless ISP (WISP) beams a 2.4 Ghz 802.11 signal to my rooftop antenna (line of sight)
• Rooftop antenna sends that signal to the router (i.e., radio) inside the antenna enclosure
• A power-over-ethernet (POE) cable powers & connects that radio/router to the house (P + Data)
• Inside the house, the other end of the POE "Y", i.e., Data, connects to the home router
• That home router "serves" the entire household on the Internet

Temporarily, for a week or two while a new antenna setup is being erected, I'm connected at a SINGLE LAPTOP (Ubuntu 10.04) via a portable antenna hooked to a portable USB router (Amped UA600 500mW 5dBi Wireless Router) tied to the USB ports of the USB laptop.

That works fine for the one laptop, with the only wires connected to it being the USB ports (i.e., the RJ45 is free).

This is the temporary configuration:

• WISP signal is beamed to the portable antenna
• Portable antenna is integrated with the portable router (P + Data) where power & data is via two USB ports on the laptop
• Only one PC is on the Internet

QUESTION:
How do I use that RJ45 on the Ubuntu laptop to "serve" the home router (WRT54G)?

[rocksockdoc]

• Setting up an Ubuntu Wired/Wireless Router
• Turn your laptop into a wireless router in Ubuntu
• Setup laptop running ubuntu 10.04 into router
• use laptop as wireless router
• make my laptop a wifi router
• Ubuntu Laptop as a wireless hotspot on bridge mode
• Make laptop a wifi router and share internet from ethernet or a CDMA 3G USB dongle
• Is there any way to turn laptop into wifi router/hotspot