very simple. masked a line with tape, cut gently along it with a sharp xacto. kept cutting along that line, deeper each time but not forcing it. about five passes did it.

used a grip to put the cut piece against the other side and cut along that to get the line, then same process.

xacto blade is good because you can also use it to put a slight bevel on the edges, as i did.

then shoe polish to blacken the exposed edge.

very, very easy. maybe 10 mins tops start to finish