The best way to have a good idea is to have lots of them.
I made 11 pair of felted slippers for my family for Christmas. Three of them are not pictured because I sent them off to Michigan before I took a picture.
They are made from felted wool sweaters which I purchased from thrift stores or were given to me by family members. The pattern is on Martha Stewart's website.http://www.marthastewart.com/good-things/felt-slippers They have a template which I took to the copy store to enlarge to the various sizes I needed.
It was fun to decide how to embellish them. A lot of work, but a fun project.