Pressure washing my garage floor, need something to protect the skirting what would be my best option
Welcome to the Bunnings Workshop community @lmoores. It's fantastic that you've joined us, and many thanks for your question about protecting skirting.
The issue I see is that obviously you'll want to clean right up to the skirting board. This would mean taping to the bottom edge of the board, so all the concrete is exposed. What's going to happen is the pressurised water will penetrate up under the skirting through the gap at the bottom edge. This isn't going to be great, especially if the skirting is MDF and not timber. You could caulk this gap with Selleys 475g No More Gaps Interior Multipurpose Gap Filler to minimise any water penetration. Make sure you allow it to cure before pressure washing. Even then, I would try not to point the pressure sprayer's nozzle directly at it.
Another option might be to get a piece of thin board and use it as a deflector. Make your way along the wall placing the board against the skirting where you are cleaning. This will mean the 3mm thickness of the board is still not cleaned, though it should deflect most of the spray. Make sure to keep the nozzle perpendicular to the floor and try not to shoot under the board. It would be best if someone follows along behind you and mops up the excess water before it has time to seep under the skirting.
Let me mention @TedBear to see if he has any other ideas.