How do you rewire a NEMA 5-20 plug with a standard 3 prong plug?

A NEMA 5-20 plug is a standard 3 prong plug. It wires the same as a NEMA 5-15 plug (Ground wire to the "D" contact, Hot to vertical terminal and neutral to the horizontal terminal (with the Ground terminal either at the top or bottom)).

A 5-20P plug has the neutral blade rotated 90°. The 5-20R receptacle usually has a T-shaped neutral hole, to accept both 5-15P and 5-20P plugs.