Or, if you'd like pictures with your tutorial (because Eric beat me to the punch...), here is my contribution.
This panther wants to gain a hull down status with respect to the ridgeline some 700m in front of it - note that the ridge it is behind blocks it's view of it's "target"
So, give a Hull Down order, placing it on the point (the top of the target ridgeline) you wish to be hull down to.
....and hit "GO!" - the Panther trundles forward, stopping when the waypoint comes into view - voila! Hull down!
Some common sense has to be applied when using this command - it is silly to try to go hulldown to a point behind a stand of trees, for example - the unit will never gain line of sight to the waypoint.....and you will bash you head against the nearest wall....
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As an aside, I believe there is a bug concerning how the HD status of units is calculated which makes gaining a "Hull down advanatge" over enemy unit very difficult. In almost every case, if you are hulldown, so is the enemy - it is next to impossible (I have found...) to gain a HD advantage from a ridge - it is a bit easier from a hollow, but not much....