If you haven’t got one of those templates yet, check out [Guy Perez]’s design for an adjustable dovetail template that’s easy to produce with a 3D printer. It consists of ten separate pieces ...

For example here is the net of a cuboid. The nets of 3D shapes help you see some of the properties of the shape. This net of a tetrahedron shows you the shape of each face and the number of faces.