There are two ways to do this. Either by turning your phone into a hotspot, or by tethering via a USB cable. In your wireless setting on your Nexus you'll find this screen:
Selecting "Portable Wifi Hotspot" will let you configure your phone as a wireless access point, you can then join your laptop to this access point just as you would normally use wireless. Make sure you configure the access point to use WPA in the "Portable Wifi Hotspot Settings" so you're not inadvertantly providing everyone around you with free wireless on your data plan!
The 2nd way is to plug in your phone into your laptop via the USB cable and then selecting "USB tethering". Ubuntu will think your phone is an ethernet connection and just connect automatically.
I've done both, but given the option I use the tethering more as having both radios cranking on the phone can go through battery quickly, when you tether at least the phone stays charging.