The hardest part about this knot is making sure that somebody is watching while you do it. Every time I tie my shoes alone, I feel an upwelling of regret, as if there has been an opportunity lost.
grab the inside of 2, invert, shove into the inside of 1.
do the same for 3 into 4.
now you have a sheet folded in half, with two pairs of corners together.
grab the inside of 1+2, invert, shove into the inside of 3+4.
now it's folded to a quarter, and all the messy stuff is together.
lay out flat, straighten messy stuff a bit, so you can do the next step more easily.
grab the whole thing, chuck into the closet.(I do fold sheets for the guest bed but that’s because the alternative set might spend months on the shelf. Most of the time the same set is washed and put straight back on the bed.)
(btw I use Ian's knot)
But let's say your washing process is different to mine. I'd still rather do two trips upstairs than fold sheets.