When I do a quilt from a kit I go to Joanns and get material to match what I have done. Sometimes its only polka dots or stripes to match the colors. 1 1/4 yard usually does the trick. A lot of times I find babys for a baby one or bears what ever is on the front. Wild Thing that I did I found a jungle print that matched almost exactly.
I pin the right sides together and sew around the edge. Trim off the excess and turn it right side out. I then stitch around the edge again.
This makes for a nice back to compliment the front and covers up all the stitches on the back of the blanket. One blanket I had had a lot of seperate x's on it so when I got done I took yarn instead of doing the x's and tied knots there and also made another on the back side after I got the backing on. It was really cute.