An easy way to sew a mitered seam in your binding strips is to let the ruler and the mat do some of the work for you, so I've lined up one edge of my binding strip on one of the inch marks on the mat and the other crossing it with that 45 degree line intersecting where they cross here and here and of course the fabrics are right sides together. Now I'm going to place my ruler over that and line up my quarter inch mark with that 45 degree line on the mat, then I'm going to take my rotary cutter and cut through both layers. Now I've got the appropriate amount of overlap so when I sew a quarter inch seam I'll be sewing it along that 45 degree line and everything will line up. So now I'm going to keep those lined up with those little tails sticking up and as I sew with a quarter inch seam, right along that bias edge, now you don't want to pull too much on that edge, that bias will stretch, that's where the most stretch in the fabric is so you want to make sure you're not pulling to hard. So as you can see because of the way we lined up and cut that top edge is straight, and the bottom edge is straight. Of course we need to nip off that little point off with some scissors. But the reason that you do a diagonal seam in your binding because if you did a straight seam and then you folded your binding in half to apply it you'd have a big lump of all the seam allowance in one place on your binding, with this diagonal seam that seam is much more spread out so as you're bringing it around there won't be one place where all those seam allowances are. From here when your attaching binding of course you press it with the wrong sides together and attach is as always.