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