SUPPLIES:
-L Square or Ruler.
I recommend an L-square like this one; it makes getting that perfect 90 degree angle a breeze.
-50-100 ft. of Grosgrain Ribbon in the Color of Your Choice. You can get this ribbon at many craft stores, but I love this selection on Amazon.
-Tacks in a Matching Color to Your Ribbon (100 ct.) (I was using black ribbon, so I got these matte black tacks from Decotack.)