Josh
I start by marking everything out in pencil. I don't necessarily draw in every line, often it is sufficient to just mark the spacing out either side, or end, such that they can be spanned by a twelve inch steel ruler. I line the steel ruler up on the marks and hold it as firmly as I can against the work. I have a small wooden handled triangular file. The tip of the file is squared off into an equilateral triangle about 2mm each side. Plasticard can be easily shaped, I push the tip of the file, gently at first, along the side of the ruler. Depending how deep you need to scribe, depends how many passes you need to make, and how hard you press. You are really carving out a 'V' shaped channel. I occasionally run a fine file across the tip of the triangular file to keep the edges nice and sharp. Given that the slabs are supposed to be stone, I take a scalpel blade to a few of the edges, to represent chips and breaks. The odd slab has a few light scores across to represent cracked slabs. I hope that helps.
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View attachment 359586
David