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The same polyhedron changes completely depending on how it rests. In the exam it is common to ask for the tetrahedron or the cube in a specific position: you must deduce the plan before drawing the elevation.
The simplest position: the base is a square in true size in the plan and the vertical edges rise to the height of the cube.
Practical rule: the height of the polyhedron in each position is a known length: the edge a (resting on a face), the diagonal of a face a√2 (resting on an edge) or the diagonal of the cube a√3 (resting on a vertex). Work out that height before you start.