The tutorials below, authored and narrated by vera viana (11.2019), show an example on how to model* the five convex regular polyhedra: the tetrahedron, the hexahedron, the octahedron, the dodecahedron and the icosahedron.
Regular hexahedron (cube)
rhino’s command box allows you to model a cube quite easily. Just select the tool, locate the point in which the cube is to be drawn (first corner or base), type the edge lenght, type enter for the width and enter again for the height. this will ensure that every edge lenght of the box is the same.
* the software used is Rhinoceros (version 6.0)