How the hell do I ambush someone?

I only tried it once. A large Danish army was moving towards my city, I had an army near by and figured I'd intercept. There was a lot of forest near by where he would have to pass so I figured I'd try to make it an ambush. Parked my army in the trees and hit end turn. The Danish army just went straight past and seiged my city.

I tried reloading and moving my army to a different part of the forest but the Danish army walked right past again.

What am I doing wrong? Is there a trick to it?

Also, I've been ambushed a few times in M2 but never successfully, how can I make sure that a planned ambush is successful?

The army needs to move right on your ambushing army, it seems to me that it rarely happens once theres roads every where. I have had 3 ambushes so far and once in my haste i did not pay attention and ambushed my allies haha