How to enter NPC Zones

First
Form an army. 

 
Second
Click on your army, and you will see an option saying, Transfer to an NPC zone.

Only with a transition crystal can your army be teleported to an NPC zone. 

 
Third
Choose the NPC zone you want to enter. 

Fourth
If you want to withdraw your army from the NPC zone, you just need to click your army and choose the return option.