While working at the department store that I worked in 20+ years ago in Illinois, I often had to re-tie sashes on dresses that came untied for whatever reason. I learned that tying a sash bow is a little different than tying your shoes. At first it felt all backward and upside down, but I soon got the hang of it.
1. Place the right hand side of the tie over the left.
2. Take it under the left hand side and back over the top again.
3. With the left hand side, make a loop with the end of the sash toward you and the loop bent back toward the left side.
4. Place the right hand side over the loop.
5. Pull a loop of the right hand side up behind the left hand loop.
6. Tighten it up and there you have it.
Pretty simple really.