Question 152104
You start by carefully folding down one corner of the 8.5 by 11 sheet of paper so that the top of the sheet is accurately alligned with the side of the sheet. 
Crease the paper at the fold so that it maintains its shape (a triangle).
Now you fold up the bottom of the sheet (this has the shape of a rectangle with dimensions 8.5 by 2.5.  The 2.5 comes from the 11.5-inch side minus the 8-inch side that was folded down in the first step.
Now you have a side of the twice-folded sheet that measures 6 inches, if you have folded very carefully and if your sheet was really 8.5 by 11 inches.