Dispatching an Expedition. 

A plundering mission's carrying capacity or Load is low. Make sure you have sufficient carrying capacity before dispatching your troops on a plundering mission. Your troops cannot carry resources that exceed their Load value. 
 
Click on the Muster Field or go to the World Map and click on a target (Wildlands or City). Then decide what the goal of your expedition will be - Occupy, Plunder or Scout. After that, the Expedition menu will appear. 
 
An army must have troops, a Hero, and Food. For a Scouting mission, you need only Scouts and Food.
 
Troops on an expedition will consume Food. If you plan on going far away, ensure that you include units with a high Load value (Peasants or Transport Wagons) in your army. Otherwise you may not have enough Food to maintain the troops for the duration of your expedition.