The methods not that difficult, cut four pieces of stone exactly the same width and length (the lengths not to important as you can always trim the vase at he end) put internal mitres on both the long sides and the bottom of each piece.
Mix your resins to match the stone, and glue the four pieces together at right angles to make a the body of the vase.
Using this as a template, place on a piece of stone and draw around to give you the size of your base, cut this piece and put an internal miter on every side, mix your resins and stick to the body of the vase.
You now have your vase, all you have to do now is polish up the mitres, chamfer the top etc, the finished result will depend on how good you are with a flex.
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Here's a photo of an external corner in one of my bathrooms, if someone can flip the picture, much appreciated.