Use a piece of wood to cut up against or use a piece of string and camping pegs to mark out the line, then cut along it with half moon. Or if on a curve, use a white chalk spray and draw it out, then cut along the line.
Piece of straight wood (or string if a long edge) and a cheap serrated kitchen knife. I have half sunken bricks all around mine. Costs a fair bit and takes a while to do but once it's done you never have to edge the lawn ever again! Just strim along the top of the bricks.
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For the straight sections I lay a piece of 4" x 1" timber and cut along that. The curves I mark out and then cut carefully.
I live in west central Scotland - not where that photo is...