Start by picking the light switch that you want to decorate...
Then grab a piece of plain paper...
Place it on top of your light switch...
Then feel with your pencil, the outside of your light switch and trace it onto your piece of paper, also trace where the switch is...
Then cut out what you traced...
Decorate your paper, and then grab a glue stick, and cover your light switch with glue and then glue your decorated paper down (make sure that you don't glue the actual switch itself) ...