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I don't know, I do it all the time all by myself.
Maybe I missed something?
But then, I can't play guitar so I guess it evens out somehow!
It's encouraging that someone out there knows how!! 
You take the corners of one end of the thing and stack them, one inside the other and sort of drape the corners over one hand and hang on to that corner. With the other hand, find the edges from one side of the corner, holding them together, lightly run your hand down the edges* to the other two corners and do the same thing with them. Now drape all the corners over your hand and line up the edges with the other, find the center and fold that to the point of all the corners, not the edges of the fabric, you will have excess fabric for a moment. Now lay the whole thing on a flat surface and smooth the middle of the sheet fabric and make sure you have the edges together along both edges not worrying about the corners for this part. Now take the weird corner part and one of the edges and fold the excess part in at the edge so that you now have a straight fold. Do this with the other edge so that you have a realistic looking rectangle, fold into a tidy rectangle/square from there. All you are doing is lining up the fabric so you can fold in, like flaps, the excess parts that go down the side of the mattress in so you have a shape you can fold neatly.
Takes a little practice and sometimes I have to start over because I had the wrong corners together or one end was inside out.
* I usually hold them with my index finger in between to smooth it all at the same time and lifting and lightly shaking the fabric below helps it all line up. You don't have to go the whole length all at once either, I do a gathering thing where I straighten out a foot or two at a time.
Try it, you'll like it. 
I will keep those instructions until the next time I change the sheets and see what happens!
And let me know if my instructions work! I've never tried to explain it.
That's way easier than my method! 
...I get up to pee, and there it is, lying haphazardly next to my head, no matter how carefully I've fitted it before I go to bed. Considering this...there's no way in hell I'll *ever* be able to fold one like a normal human being...
