With the rubber collar on the end of the Pelican HydroClean™ Pond Skimmer's siliconed inlet pipe - pull any wrinkle in the rubber liner from under the rubber collar towards the skimmer's body.
Most of the wrinkles will come out, but a few small wrinkles will remain. This is normal. When the Stainless Steel Hose Clamp band is tightened down, the rubber liner itself becomes a water tight gasket, plus, the silicone will dry and create a "second layer" of water proofing.

With the rubber collar holding the liner to the siliconed Pelican HydroClean™ Pond Skimmer's inlet pipe, install the Stainless Steel Hose Clamp band over the collar.

DO NOT TIGHTEN YET!
When installing the Stainless Steel Hose Clamp band onto the rubber collar holding liner to the Pelican HydroClean™ Pond Skimmer, make sure the Stainless Steel nut is near the top.
Having the nut on, or near the top will make for easier tightening.
With the STILL UNTIGHTENED Stainless Steel Hose Clamp band on the rubber collar, and the rubber collar holding the rubber liner onto the siliconed Pelican HydroClean™ Pond Skimmer's inlet pipe - now you're ready to cut a 7" hole in the liner.
Using a razor knife, cut a 7" diameter hole in the "middle" of the inlet pipe - about 1½" from the inside perimeter of the 10" inlet pipe.