"My quilt instructor suggested cutting fabric doubled once and held up selvedge to selvedge and then moved back and forth so the fold lies even."
To me this is important step #1. If the fabric is not folded on the line of the woven thread it can never appear straight when cut. It is easier to work with smaller pieces rather than 5 metre lengths however. Take the fabric, gently press, starch if you wish. Take hold of the selvedges right and left and bring them together. (Newbies remember we are cutting across the width of fabric ,the selvedges run the length.)Hold the folded fabric in front of you , the folded edge hanging down, then manipulate the selvedges back and forth until the fabric falls evenly without a wave or buckle to be seen. Lay it on the board with the fold closest to you . Then line up the ruler on the fold line VERY carefully. Cut to square the fabric, then cut your 1st strip. You may need to move to the opposite side of the table to do this accurately. CHECK THAT THE STRIP IS NOT BOWED! Repeat the whole process if necessary! Moving the fabric to the mat can sometimes mess with the selvedges again,be careful with this step, be patient !
Remove the strip, do not move the fabric piece until you have run out of room to cut. Realign cutting edge every 3-6 cuts, but keep checking your strips for accuracy. Always sew long strips in opposite directions to keep from creating bowing after sewing. I hope this helps a little.