I see what you're saying now. I thought you were referring to the fittings coming out of the skimmer.
There are a few ways to do it, actually. The safest would probably be to let your cutting be the last thing you do in case there is a chance the side of your pool distorts during the cutting. This is the route that many here suggest, and it also means that you'll have to split the gasket in half which I didn't want to do. I figured that since it came as a single piece, the manufacturer thought it should be installed as a single piece.
That being said, I went a different route. I marked where I wanted to cut it using the skimmer trim ring for the inside of the pool and a felt Sharpie, filled it to a few inches below that, and then made the cuts about 1mm outside of the pattern. As a side note, I didn't see any distortion in the lining after I made the cuts. In one of the pictures (
http://s1332.photobucket.com/user/w...ia/Pool Skimmer/IMG_1169_zps8131b7ff.jpg.html), you can see my initial mark for the gasket. Thankfully though, I realized I had marked it too high before I had made the first cut. You have to take into account the seam on the inside of the liner.
Once I had the hole cut, I fitted the gasket and poked holes for the bolts using a small philips-head screwdriver. Once you have the initial holes made, you may notice that you'll have to use a bigger shaft screwdriver to make the holes slightly bigger, but not too big. I wanted mine to be a tight fit to try to minimize the possibility of leaks. After everything was cut and holes made, I grabbed a helper to hold the skimmer and tightened the bolts up a little bit at a time so as not to over-tighten one and under-tighten another.