Just cut square-ish pieces of paper from your pile of used wrapping paper. any size. Last year I made a snowflake that was ~2 feet in diameter. (That was virgin wrapping paper, never wrapped a present at all. In my green defense, it was the last bit on the roll, and I knew we were moving - I was not going to move 2 square feet of wrapping paper. Instead it decorated our glass door until Valentine's Day.) I also made really small ones, and all sizes in between (with the used wrapping paper, of course).
Fold up and cut like you do with normal, computer paper-white paper. Then tape in your windows (walls, mirrors, etc.) and enjoy. Last year our windows were adorned with such a mix-match of different papers, it turned out so cute. And Aubrey really enjoyed helping make them. Lauren helped as much as she could. Andy might've even made a few. I don't remember.
This year (we won't be home at Christmas time) I'll be folding up pieces of wrapping paper and stashing them in my suitcase, for a snowflake making extravaganza when we get home!
Okay, and this green tip is kind of a stretch. I know that. But it is really fun to make them, and, seriously, the patterned wrapping paper turns out such cute snowflakes. So much cuter than boring old white paper. I'll never go back.