You need one of these. Connect your vacuum hose to it and set this in the skimmer opening.
If you have control of each skimmer you can turn off the other skimmer. If not, then try to use the skimmer closest to the pump as that one would tend to have more suction.
You could put a tennis ball in the unused skimmer to stop the flow. That's what I did while waiting to be able to use my robot. Also, my PB didn't supply me with the skimmer vac plate, so I just put the end of the hose in the skimmer hole, and that gave me enough suction.