I'm not used to give istruction ready to go, I like to give some suggestion to put the other users in the way and let them to refine the instructions with a bit of googling.

BTW.

Install the power kernel and boot with it (search the forum an or the wiki)

Install iptables

#2 unload any g_nokia and g_file_storage file modules eventually present

rmmod g_nokia

rmmod g_file_storage

#2 load the module g_ether (and install the related driver in windows, read usb_networking howto to know where the driver can be downloaded )

modprobe g_ether

#3 bring up and configure the usb0 network adapter

ifconfig usb0 up

ifconfig usb0 192.168.9.1 <--- (that ip is just an example)

If you want to share the 3g connection you ave also
to

#4 enable the ip forwarding

echo 1 > /proc/sys/net/ipv4/ip_forward

#5 set the iptables roule[s]

iptables -t nat -A POSTROUTING -o gprs0 -j MASQUERADE

#6a configure statically the ip/gw/dns on the pc side

Following the given example configure manually the rndis lan on windows as follows

ip 192.168.9.2

gw 192.168.9.1

dns (any working dns eg 4.2.2.2)

#6b or (better) configure dnsmasq on the n900 to act as DHCP/DNS server.

sometyng like that on the n900's terminal

dnsmasq -I lo -z -a 192.168.9.1 -F 192.168.9.2,192.168.9.50

Now you can browse the internet from your pc.

You may want also to use the opposite way: share the the landline connection to your n900, this is already explained in the message 6

My experience trying to share my PCs LAN connection with my N900 over USB was a nightmare but it eventually worked.

Using MAD Developer;
Click on Manage USB
Click on Load G_Ether

Windows (XP) will either create a new LAN new connection using the installed LINUX USB Ethernet Driver
OR
it will detect the new NIC ask you to install the driver , see guides for completing this step.

The above steps are the same as the cmdline steps in #1 and #2 .

I perform step #3 on the N900 but with IP addy 192.168.2.15
I now have
192.168.2.14 -> Windows XP USB NIC
192.168.2.15 -> N900 UCB NIC

From each of these hosts I can ping the other host. You could now establish a VNC connection to your N900 if you have the VNC server installed on your n900 and a VNC client on your PC.

To browse the internet on your N900 you still have some act
ions remaining – primarily sharing your PC LAN/WAN connection – clearly a direct connection to the internet or connected via a router.
I’m using NAT32 Windows Software router to avoid using windows ICS which has a lot of compromises.

Hope that helps.