A circular saw with the blade turned backwards is the easiest way. I have a skirting jig I made that I will post pictures of tomorrow, but it is not really necessary for one skirting job. You can take any 2 pieces of scrap plywood or whatever you have that is wide enough, at least 1/2" thick and flat. Set the saw blade to 1/4" deeper than the skirting and support the cut within an inch or so on the right side. Support the left side also, but it does not need to be as close. The shoe or table of the saw will be supported by the flat object on the right side.
Small cuts, pipe holes and the like can be cut with snips or you can score with a utility knife and break off pieces or punch out holes.
I have used the cardboard box the skirting comes in (supported on the inside with scrap), 2x4s, plywood, and two folding tables before. Just about anything can work for the work area. If you have an area of grass that you are not all that fond of, that could work too if it is flat.
Today is PERFECT!
All information and advice given is for entertainment and informational purposes only. The person doing the work is solely responsible to insure that their work complies with their local building code and OSHA safety regulations.