(Quilt 'sandwich' on the design wall.)
There are lots of ways to make a piece of fabric bigger to fit your quilt. I often find that I have a piece of backing that I want to use but it's not quite big enough. So here's what I did on my bow tie quilt and it's very similar to what I usually do when piecing a backing.
PLEASE NOTE: I only make my quilt backing 2'' wider and longer on each side of my quilt top because I machine quilt on my Janome and don't want the extra bulk of batting and backing. If you are sending your quilt to a long arm quilter, your backing will need to be at least 4'' wider and longer on each side. Or check with her to make sure.
Here is a photo of my quilt top pinned to my design wall. I take the fabric I have, and cut it the length I need to extend past the sides of my quilt by 2'' on either side. I then took the remaining length of fabric and cut it in half. This shows me how much space I need to 'fill in'.
I used a strip of border fabric to fill in the gap across the backing. I then had to use two pieces to fill in the remainder between the two smaller pieces of backing. Remember to add seam allowance to your pieces.
I use a quarter in seam allowance and press my seams open. I sew the pieces together and either back stitch at the edges or add additional stitching to keep the seams from popping open at the ends like shown above.
Once I had the backing sewn into two sections. The long strip to the large piece and the smaller pieces sewn together, I cut the right side of the top section down the middle. It would have been fine to leave it the way it was, but I think it looks better when it isn't so symmetrical.
Here's the finished backing. I sewed the cut off piece to the opposite side of the section, then sewed the two large sections together. If you look closely you can see the seam. Now my backing is just the right size for my quilt and I didn't have to go buy another backing!
I then pin the backing to the design wall, and piece the batting! I used large sections of leftover batting and just fit them together until they covered the backing. I then trim the edges even and sew them together with a large zig zag stitch. I take a lint roller to the finished batting and smooth it out over the backing. I add the quilt top, smooth, then pin baste on my design wall. Or go here for instructions on how to spray baste a quilt.
I'm sure there are dozens of ways to do this, and I don't claim to have the best method. It's just the best method for me. I hope it works well for you too!
Enjoy every stitch,