I need to make a short extension cord that goes from my portable
generator 240V 4-wire receptacle to a 3-wire receptacle that powers 2
120V house circuits. The 3-wires are red and black and white. How do I
make this? The 3 wires go into a junction box where the white wire is
attached to some kind of common connection and the red and black are
each attached to its own toggle switch that flips the power between
the power company and the generator.

TIA