Graph theory for Networks
The way this works is you go through the diagram and label each current and voltage. Each current needs a direction. You can pick any direction for the currents that you want. 

You might get bunch of equations that look like this, for the resistance sections:

\$ \frac{V_{nodex}-V_{nodey}}{R}=I\$

Notice that 

\$ \frac{V_{nodex}-V_{nodey}}{R}=I\$

is the same as this (nodex and nodey are reversed below, hard to see):

\$ \frac{V_{nodey}-V_{nodex}}{R}=-I\$

So as long as you remember to keep things consistent by correctly labeling and using the same direction if the label for the equation, you'll get the right answers.