I just look for a forested "square" on the campaign map that's between the enemy and its target, usually my settlement. It helps if the ambush site is next to a road, meaning the enemy is most likely taking that route. If the enemy walks into the ambush, I deploy the troops depending on the situation. You can see the enemy marching column in the deployment phase, so it'll help a lot. The most important things are that the enemy has its cavalry on the front and the general is on the front left.

Here's a quick illustration:


The dark blue lines are my elites, the blue lines are my regular troops, the cyan lines are missile units, the red block with a yellow dot is the enemy general and the huge face is my general, preparing for his bloodbath.

Also, in these situations I don't let my troops throw javelins before charging. If you let them, the enemy will have enough time to organize.