Windows 7: Problem with internet connection sharing. Help needed.

I have two computers running Windows 7 Ultimate. They are connected using a wired Linksys router. One uses a 3G wireless modem connection that i would like to share with the second computer. I have tried using the tutorials on this site and others with no luck, possibly because I'm new to computers and don't understand the terminology. I have managed to get Remote Desktop Connection working and have shared files and printers successfully. Any help will be greatly appreciated. I also share this connection with my Xbox 360 with no problem.