When I bought my 1st pair of poles 3 years ago, I had no clue of how to use them or set the length of them.
I found that this video really was helpful on showing me how to use them with the wrist straps to help propel me up and down the hills.
It is pretty comprehensive about selection, use, and care of the poles.
Hope this help you out.
Wouldn't go on a hike without them.
Also, to answer your question, per this DVD, the top of the poles should be placed about the height of your hip.
The middle section should be about the width of your fist and the lower section locked at a length that places the top of the handle/pole at your hips.
When you are a down hill slope, you can then adjust only the middle section to be longer to help slow you down or to ease the force on your knees.
While going uphill, you would shorten the length of the pole so that you can still easily swing the poles uphill without prematurely hitting the uphill slope.
Use of the straps is critical as you are using your wrist/arms to carry some of the load of your body as you step up or down hill.