Iron the fabric. Match up the opposing selvages (this is how it came off the bolt). Stroke out the fabric so it is flat. Lay the folded edge part of the fabric (where it is folded on the bottom) so that it lies along the selvage edge. The fabric is now 4 layers thick.
If you are right handed, take your ruler and line it up along the left edge of the fabric and cut a strip to make the edge clean (right side of the ruler). Usually this will be about a half inch or so wide. Make sure you have a straight cut from top edge to bottom edge. The ruler will be overlapping the fabric but most of it will be on the table.
Remove the initial strip you just cut. Place the 4 inch measure on the ruler on the clean edge you just cut. Cut along the right edge of the ruler(which is laying on the fabric) so that you now have a 4 inch wide strip.
Take that strip and rotate it (or position yourself) so that it runs 4 inches tall and the long side is horizontal. Again, cut a strip on the left end to get a clean cut. It might be .5 inches or so. Lay the 4 inch measure on the ruler on the left edge of the fabric and cut along the right hand side. You'll have four 4 inch squares. Move the ruler over another 4 inches and cut for another stack of squares, etc.
If you need more, cut more 4 inch strips from the fabric.
Does this make sense?