Katherine put together these awesome tutorials and I followed them to make my own muff and cover. I have to say that making the covers is pretty satisfying since you can make them relatively quickly.
Like Katherine, I uploaded a portrait to Spoonflower and had it printed on silk. I actually was able to have the portrait tile so that I was able to get more than one on a single test swatch.
This project was also great because I managed to use things from my stash, which included muslin, cotton batting I had leftover (and I had to pull it all apart because it was flat and not the loose stuff), cream dupioni, flat gold sequins, and cream silk satin ribbon.
The portrait I used is one of my favorite portraits, "Portrait of Mrs Russell nee Cox 1781" by John Smart. (Larger version and more detail here.)
Here are Katherine's tutorial links:
Muff base: http://koshka-the-cat.blogspot.com/2013/01/an-18th-century-muff-base-tutorial.html
Muff Cover: http://koshka-the-cat.blogspot.com/2013/01/an-18th-century-muff-cover-tutorial.html
Decorating the Cover: http://koshka-the-cat.blogspot.com/2013/01/another-muff-cover-mini-tutorial.html
And here's how mine turned out. It's not perfect, but I like it! And I want to do another!
I actually later upgraded to one of The Lady Detalle's muffs, because she makes really down ones for a totally reasonable price, especially for the amount of work they take to make. It's way nicer than my polyfilled one! https://www.etsy.com/shop/theladydetalle
For my own future reference, this muff cover is 13.5"x 25".
The cover before decoration: