if you want to cut your borders lengthwise you need to carefully fold your fabric into a manageble size- make sure you line up the folds well- trim the edge so it is nice and even- line up on the folded edge and cut your borders to the width you need-
or you can use a ruler with your fabric opened up and using chalk/pencil/or some other marking tool and draw the line the width you need then cut on the line with scissors