Flint tends to act just like glass when you hit it. The force of hitting it wants to chip out a cone-shaped piece on the other side from the hit. Kind of like when your BB gun pellet hits your mother's kitchen window and pops out a little divit on the inside.
Usually, you just tap down right along the top side of the flint edge at around a 90 degree angle, to pop out little flakes on the underside. If that flake pops off far enough up to the top side, you end up with a sharp edge. If it's not sharp enough, you can flip it over and try to knapp a flake or two off of the other side to thin down that edge.
The pliers allows you to grip along the edge, and then press down a crack off a bit of the flint. You can also use the handle ends to press down on the top of the edge of the flint to pop off a flake from beneath it - instead of hitting it to pop off a flake. You get a little more control from "pressure flaking" than from "percussion flaking", but pressure flaking only works with thin edges on the flint. It is usually used to touch up an edge on your flint when you are making an arrowhead, knife blade, or gun flint.
Do a web search for Flint Knapping. There should be a few sites that show the steps to knapp an edge. And there are a number of good books and videos out there on Flint Knapping.
One of the problems of having/using a traditional flint striker is getting/keeping a sharp edge on your flint. I've got a couple 5 gallon buckets full of flint chards, and I'm still always having to knapp new edges on the pieces of flint I usually carry. It's just one of those tasks/skills you have to learn and do.
I hope this helps. Of course, these are but my humble thoughts to share.
Mike Ameling