Choose either blue plastic tubing for the heroic Jedi or red for the evil Sith. Measure the circumference of your flashlight -- that should be the width of your plastic tubing. The length of the tubing should be about 3 feet long, although you can make it longer or shorter depending on how long you want your lightsaber to be.
Lie the tubing flat on the ground and place the flashlight on the bottom. Roll the plastic tubing around the top of the flashlight, making sure the on/off button is still easily accessible. Use duct tape to attach the tubing to the flashlight. If you need to place a few pieces of tape along the length of the tubing, make sure to keep it thin and small, as it will be seen when you turn it on.
For the top of the lightsaber, if you can't bend the excess plastic tubing to meet your specifications, simply take a small piece of Saran wrap (red or blue, depending on the color of your tubing) and wrap it over the top. Attach it with invisible tape or a small amount of duct tape. The important thing is to keep the white light from the flashlight from shining through.
Turn it on, and you have a lightsaber of your own. And don't forget, as you practice your Jedi training, to add your own "voom" sound effects!