I started by measuring and cutting the blanket on the outside of the box, this way I was sure to know that it would also fit on the inside of the box. By the way, I love duct tape, for all kinds of things, this time it just happens to be holding the blanket in place while I work on the other side of the blanket.
I cut two long pieces, one for the inside and one for the outside, that fit from the top of one side of the box across the bottom and up the other side. Then I cut squares that fit each end of the box, again one set for the inside and one for the out.
Then I repeated almost the same thing for the outside except I folded in the rough edges to give it a more finished look. Yep, again with the duct tape.