gram2five

I just finished this top. It will be a Christmas gift for my DD and family. It is a throw size for their sofa.
I would like to do a pieced backing, either rectangles or stripes, but I can't seem to wrap my mind around how to do it so everything is even on the back because of the extra 3 o 4 inches you are supposed to add when you put the sandwich together. I'm sure someone on here will be able to explain so this newbie can understand. Thank you!

The second picture are the colors I'm thinking about using for the back.