We did this a while ago.
We don't have widespread recycling here in our new place that there is in Oregon, so we have to get more creative in the way we "reuse" things. Like milk jugs.
Make sure it's cleaned out well. Sour milk smell is hideous.
Cut out the handle and spout with some tough scissors or a craft knife. If your child's head is small, like say toddler size, this is all the cutting you need to do except for eyes (you flip it upside down and the base sits nicely on the top of the head - no straps or elastic or strings needed to hold it on). If your child's head is larger, cut the jug in half.
Cut out eyeholes and little slits on the sides to attach string or elastic. Use permanent markers to draw the face or glue stuff like feathers, bits of wrapping paper, or whatever else you have.
These are some I did to give the boys ideas. I thought pipe cleaner worked well for whiskers. Here's my bobcat.
I show them cute animals and they choose to do...a skull with a bloody nose.
And the witch doctor.