It will depend on where the bubbles reach in the neck. You'll probably want the water level somewhere between the first black ring and the base of the cup. I would start around there or maybe a little under that. Give it some time and watch how well the bubbles get lifted into the collection cup. If there aren't quite reaching the collection cup you may want to raise the water level in the skimmer a little more. Some like a real watery skim, Some like thick. It just depends. Basically, watch how it overflows over into the cup.
[video=youtube_share;uAYTXcjZyfE]http://youtu.be/uAYTXcjZyfE[/video]
It will look something similar to this skimmer in the video. Obviously it isn't the same skimmer but the process is all the same.