strip sheets from bed. drop sheets to floor trying not to give the spooky cat a heart attack.
that's the easy part.
starting with the fitted sheet, get on your hands and knees in the middle of the bed and try to determine from the way you folded the sheet when you last washed it which ends are the ends and which ends are the sides. once this is clear, fit each corner of futon into sheet corner, working your way around. at some point before you are done you will have to climb onto the sheet; make sure that you end up on the part that is already smoothed out, as smoothing out a sheet you are kneeling on is not very easy. finish with fitted sheet.
get top sheet. attempt to determine whether selvages or hems were meant to go at head and foot since these were very cheap sheets and there are no guarantees that they were put together normally. again, on your hands and knees in the middle of the bed, attempt to position top sheet so that vertical fold is in the middle of the futon and unfold, climbing onto sheet when you are done. now unfold horizontaly, lying your body down on the sheet to smooth it as you unfold towards the foot of the bed. once you get there, check head of bed to make sure that sheets have not shifted downwards. if everything looks reasonable, tuck in end of top sheet by lifting up the end of the futon that you are kneeling on and folding the sheet under the futon and yourself without falling from end of bed.
unfold each blanket in the reverse manner from which you folded it up. much as the top sheet, smoothing out the blankets is a full-body maneuver. occasionally you might need to stop and lie flat on your back to stop the throbbing pain.
that's all there is to it, really. the pillowcases are easy as you don't actually have to sit on the pillows while putting them in the cases.
be sure and get some rest before actually going to bed. the sheets are clean. you wouldn't want to sweat on them.