There will be some kind of valve or pipe on the outlet of the skimmer like Scuba mentioned. If it has a valve, open it completely and the waterlevel will fall inside of the skimmer, then close it slowly, bit by bit and it will rise, do so untill the desired level is reached. With the adjustment pipe lower it to let the waterlevel fall and raise it to raise the waterlevel inside the skimmer body. This is how most skimmers are set, I cant comment on this particular one as I'm not familiar with them.