I knew the dress was coming and having seen the movie a few times, I had given the cape some thought. I have made dress-up capes before, but Elsa's cape is different. It flows from the upper back rather than from the neck.
Perusing google images helped me think more about possibilities. Because the cape wasn't going to be attached to the dress, I started thinking of it in terms of of a pinafore or vest. Looking at the Elsa doll's cape helped too.
We found some pretty fabric at JoAnns and I estimated I would need one yard. The fabric was 58" wide. First I cut off about 6" of the 58" width. The remaining fabric was about 36"x 54". The 36" x 6" piece was just right to go around my petite four year old's shoulders. I folded it in half and pinned it around her. The larger 54" by 36" piece I draped around her and decided to make pleats in the back and a curve under her arms. I also cut a curve removing one corner and did the same for the other lower edge corner. After marking everything, I used a lighter to carefully melt all the edges.
First I sewed the pleats onto the wrong side of the 6" wide piece for the length I had marked. Then I ironed the 6" in half and tuned under the raw edge opposite the edge I had sewn the pleats onto. The next step was to sew the turned under edge down enclosing the pleats, forming the armholes and finishing the short ends. Finally I tried it on her again and figured out the placement for the velcro. I ended up cutting it down a bit and hot glueing a snowflake shape onto the right side.