Step One: Put the blouse on and roughly outline what you want to be cut out. I use a pencil eye liner stick because it goes on really light and easy.
Step Two: Fold the blouse in half with the outline on the top side, this way you get a symmetrical look. Then cut around your markings.
(You should get something that looks roughly like this)
Step Three: Hand sew or use a sewing machine to hide the fringes created by the scissors.
And you're done!