some people drill holes and leave some of the plastic on for strength and some people cut out the whole side of the panel leaving only enough to fasten the gill vents......your choice.
Drilling holes works best keeps the side panel a lot stronger especially when lying over the sled in the snow. kit comes with a snow eliminator screen use as a template for a hole pattern. I also used some foil heat tape on the inside panel before I drilled the holes just to help out on any heat issues, 2 seasons on them all is good I run gills on both my sleds.
On my last sled i just cut the whole hole out. On the new one, i spent the time to leave ribs between each gill, but still having the holes completely open. The vent itself is easilly rigid enough for this setup. The vents are tough!