If the network is planar, then probably the simplest choice is to:

• let one of the loop currents be inside each of the meshes of the network
• let all of the loop currents be in the same direction (either all clockwise or all counter-clockwise)

This choice is called the mesh current method. An example is shown below.

Other choices are possible such as the following:

The only restrictions on choice of loops are:

• Every branch of the circuit must be included.
• It must be possible for the current in any branch to take on any value, restricted only by Kirchoff’s Current Law.