My basic idea was to have different-height strips of my colored fabrics attached at that 45-degree angle, like a quilt binding, and to have each colored strip framed by a white or off-white background.
I made blocks of completely different sizes just based on the scraps I had left and then set them in a sort of frame-like pattern on the back of the quilt.
Mainly because I had one big piece of white fabric for the middle of the back and lots of smaller scraps to make up the border, my improv blocks sat in between.
For my first two quilts, I used old cotton towels for batting.
I used warm and natural batting, just because it seems to get mentioned a lot by other quilters.
I briefly considered changing thread depending on which color fabric I was quilting on, but then I realized that would make a fairly strange multicolored mess on the back.
Some fabric stores sell extra-wide quilt backing fabric, and some people get creative by ‘piecing’ their quilt backs either in two big pieces or with some extra quilt blocks or a border on the back.