First, find an Ascot. That's probably the hardest part of this whole process. I wanted a silk one for a holiday party, and couldn't find one to my taste at any of the shops downtown. I ended up ordering one over the Internet from Italy.
If you can't find a pre-formed Ascot, this style of tie will work with a thin scarf.
Unbutton your collar and a few buttons. Place the cloth around your neck, with one side slightly longer than the other.