The only materials you'll need are your garment and an eighth of a yard of lining fabric that won't look inconspicuous if it peeks out. If you're going to be filling the slashes, have a quarter yard of lightweight contrasting fabric on hand.
Using your favorite transfer method, draw an oblong diamond shape (mine is two inches tall) onto a piece of your lining fabric and pin over your desired area. Make sure that there is at least an inch of fabric on all sides of the diamond.
Following along the guideline, stitch your diamonds.
Carefully cut out the middle of the diamond, leaving about an 1/8 of an inch on the sides. Slash each of the corners to the stitched line to make turning easier.
Next, turn your lining fabric to the inside. Press.
Optional filled slashes:
(IGNORE the interlining fabric that looked like it died in the sixties and was rejuvenated by a mad scientist. Plleeaaasseee.)
Cut a small piece of your filling fabric and pin it to the inside of the slash. I pinned mine onto the edges of each lining piece.
Stitch the edges, making sure that you do not catch the actual garment in the seam.
If any part of this tutorial is unclear, please comment or contact me so I can assist you.