Oh OK - I think that if an enemy has a spy or a general with good eyesight nearby or whatever, they can discover your army before you even get to ambush - so there's 3 possible outcomes:
1: the enemy army comes along already knowing you're there and attacks you.
2: the enemy army comes along, you try to ambush it, and fail, so you then get to attack it as a normal attack.
3: the enemy comes along and your army succeeds it's ambush - you get to attack them in their 'unready' column formation,

All the above happens without you clicking anything - all you do is go into the 'ready to ambush' pose if you end your movement in the appropriate terrain - all the above is worked out automatically when an enemy army moves into your 'sphere of influence'.

At least - that's how I think it works! If anyone has any extra info, feel free to chip in. (I mean, what do I know, I've only had the game a week! Ah, sleep, such a fond memory...)