I have been hiking for 8 years but have only just started using trekking poles.
Slow on the uptake but with the evangelism of a recent convert I now consider them essential, increasing walking speed and stability and easing the strain on my aging knees.
The height of the poles should be set so that they touch the ground when your elbows are bent at 90 degrees and your lower arms are parallel to ground.
The crucial step is to bring your hand into the straps from below and then grip the strap against the pole;
I am not convinced that reducing the pole length when going up hill or increasing it when going down hill is worth the effort unless the gradient is very steep or goes on for miles. Over the last few months I have found changing where I plant the poles is sufficient to adjust to hilly terrain.
There are many videos on-line showing how to walk with poles but I have found that the required gait is automatic as long has you keep you arms close to your body and ensure you plant the pole behind you when walking on level surfaces.