I am very new to Blender and 3D and have watched lots of videos which never show how to create a corner like the one in this picture.
Could someone tell me the best way to re-create it?
You have to select the edge, press Ctrl + B , that way you bevel the edge.
If then you want to replicate the difference in length, as in the picture, you have to select the two vertices on one side and press G two times to Edge Slide.
The vertices should slide on adjacent edges getting closer or further from each other.