Options:
[a]. Check the consistency of the slate before you proceed. Slate is porous so you will need to sift through it before determining which side to cut.
[b]. This will make it easier to create unique zig-zags and curves across the slate. The flatter the slate is, the easier it will be to make your cut.
[c]. If it is still stubborn, use a mallet to make even cuts on the opposite side. Switch the slate to a flat edge (such as on the surface of a fireplace) and continue.
[d]. Mark and cut the top of the slate when using power tools. When using power tools, it's more important that the slate lies flat on the surface you're cutting on.Choose from options above and answer: What most naturally follows?

How to cut slate
Flip the slate over when manually cutting the slate.
It's much easier to draw and cut a straight line across the bottom side of the slate. When cutting slate manually with a hammer and chisel or a handheld slate cutter, flip it over and cut along the flat underside of the tiles.
Answer:
[d].