I found the easiest (and safest, in terms of not cutting through something important) technique was to make a small starter triangular hole by making two cuts with a cutting disk (on my angle grinder) in a V shape, then lightly scoring the third side of the triangle. You can then bend the triangle up and down until it breaks off. I then cut out the rest of my openings in the floor (https://www.skoolie.net/forums/f11/r...tml#post372528
) with my electric shears (which can get started in the triangular hole) and more scoring-and-bending-off on the edges.
So the only time I was actually going below the level of the floor with the cutting wheel was to make the initial V cuts, after checking to make sure there was nothing there on the underside. There's a lot of stuff there that could get cut accidentally.