First, I match the selvages and make them even...I pin the fabric then press...then I square off the fabric...I use 2 rulers...I put my 12 inch ruler along the fold line, 12 inches acoss so I know that my ruler will be a perfect straight line...then I line up my long 24 x 6 inch ruler next to it, also placing it along the fold line...I remove the 12 inch ruler and then I cut...NOW I know I have a perfect 90 degree angle...and my strip will not be bowed... if your fabric is not perfectly even along the selvages, then you will get a bowed strip...thr trick is also to have a nice wide ruler , 6 inches at least, so taht you can really see clearly if the ruler runs along the fold line..