This shelf can be made to any size corner and with any type of timber. It's all about the space you have to work with and the look you want.
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I used an old piece of timber I had left over and first made sure to make it flat.
Next you need to cut a square. To do this I used a speed square to mark 2, 45 degree lines then drew 90 degree lines at their ends. There are many ways to get a perfect square, this is just one of them.
Measure to make sure that the shelf fits inside the corner.