How do I sew this together?
 

I pieced my back and found out one side too short so need to piece it.
The larger piece of the back looks like an exaggerated L and I need to add another piece of fabric to fill in the L. See pic. How do I do this?

Well, you could do it with the "y" seam - sew one side, then fold the fabric to sew the other seam - or you could cut off the smaller side, sew the smaller piece to it then sew it back to the larger piece. (I would likely do it the second way.) Are you going to quilt it or tie it?

I will look on You tube for instructions on Y seam....never heard of it...obviously I have never done one. Lol
i guess I will try it. Plan on machine quilting .

sewing the y seam works better when you draw the 1/4" seam allowance on. then, put a pin through the corner intersection. sew just up to that pin. now, line up the other side and sew up to the pin again. i'd rather do it that way than to take an entire strip off... less seams to show.

I'd do it the way quilting candy suggested. It is the easiest way and will be easier to quilt because your seams will be straight and lie flat.

Thank-you all!. I did a why seam - my first one- and it came out great! Thanks quilting buds !

Why can't you just cut along the top and put a strip all the way across it would be far easier.

My thought too! It wouldn't have to be the same fabric either. Or I would make the label the same size as the missing piece.

When I do a Y seam, as I get closer to the intersection I shorten the stitch length and slow down my machine. That way I can insure that I stitch right up the the intersection to stop. I also backstitch about 3 stitches to secure that point so that it doesn't pull out or move when I stitch the other side.

is there a seam anywhere in your drawing or is that a whole piece you are trying to patch to make square? If it is to be inserted....I would cut across that whole section-either vertically or horizontally, attach little piece then sew the newly pieced strip to the large piece...not finagling with a Y seam.....