internet connection sharing broadband gsm dongle

[sadhu]

I have two lm-18.3 machines connected back-to-back on a null modem cat-5 cable. I have set up different ip addresses on both machines. The mobile broadband connection is on the desktop machine, ip addr 192.168.0.114, and the laptop is 192.168.0.51. Both machines have netmask etc set to 255.255.0.0 and 0.0.0.0

The dongle was set up on both machines using the network-connections applet.

Now how do I get the laptop to access the internet when the desktop is connected via mobile broadband?

I followed https://community.linuxmint.com/tutorial/view/1163 for step 1, but died at step 2 because it doesn't make sense. I only have one eth card...


Thanks
-Sadhu