So all you need to do is measure your machine. I chose not to have closed ends on my cover. With my machine out 90% of the time, I just wanted something to cover all the buttons and needle. Ive got sticky toddler hands around here. Anyway, I measured from one side to the other. Then I measured the length. My measurements were 14.5 X 26.5 (that is with my inseam already calculated in) I added about an inch on all sides.
With a hot iron with a lot of steam, make a hem on all sides. Oh yea, I'm using a heavyweight home decor fabric. You could also use a cotton, but maybe add some interfacing for some stiffness.
Now, after I made my hem, I draped my cover back over my machine. I then measured were I wanted my ribbon ties to be. I believe mine was 4.25 from the bottom. I marked both sides with horizontal pins.
All you do now is stitch all around the cover and sewing in your ribbon ties when you come to your pins. I was lazy when it came to the ribbon! But pinning into place to begin with would have been faster.