Good luck. Make a line by chiseling a few hits and moving. Once you get a line all the way around, do it again. And again. Think of it like a typewriter. I usually pull the chisel towards myself, hit-move-hit-move ext, then go back to the start. You don’t want to be too aggressive in one spot. Even if you think it will help. It won’t. You’ll just sheer off a chunk and have a hell of a time recovering from it. If you’re especially confident, I just pull the chisel constantly, hitting it like a typewriter once I get a nice line scored into the stone. Take your time. Hit the chisel like you’re cracking a whip, not like it’s whack-a-mole. The snap is what you want, not sheer He-man power. I’ve taught 100lbs girls to shape stone. This is not going to be easy. It will take a very long time. Do everything you can to keep the corners from breaking off. You will likely fail. But that’s fine. Go rent a partner saw if it’s too hard. Cut a line all the way around it, not the full depth of the blade. Take as many chisels as you have, and start pounding them into the cut line, one every so often. Once one starts to sink and you don’t need to hold it move on to the next one, and then the next, and then the next. Relatively the same principle; you want your cutoff to come out as one piece. Don’t get too aggressive in one area. Don’t break off the corners.