How do I put a light switch on a extetion cord

You want a light switch on extension cord.
What kind of light switch do you have? You posted under GFCI device which is totally different than ordinary wall switch.
And what type of extension cord?
Add a comment with more information.

If ordinary light switch is installed on 3-wire outdoor extension cord:
Slice open the side of cord and you see 3 wires: black, white and green.
Cut the black wire in half and connect each end to brass screw on light switch.

If ordinary light switch is install on indoor 2-wire extension cord.
Look at plug and notice there is a wide blade and narrow blade.
Narrow blade is hot. You want the wire connected to narrow blade.
Cut that wire in half, but do not cut other wire.
Connect each end of wire to brass screw on light switch

If you want to install line switch onto indoor 2-wire extension cord:
Narrow blade is connected to hot wire.
Cut that wire in half, but do not cut other wire.
Open line switch.
Connect each end of black wire to brass screw. OR if no screws, then place cut wire in open grooves.
Tuck other wire into groove and then reassemble switch.