Cut out different sized circles from white paper for the main snowman. Use some brown to cut twigs for the arms. Then cut out from various colors hats, mittens, boots, scarf...pretty much anything you want to dress your snowman in. I made different types of hats for the kids to choose between.
Either help them with gluing the pieces on by putting glue on each part, or let them go to town on their own with a glue stick.
Once we were actually putting the snowmen together, I realized that my younger son had no idea what a snowman looked like. I let him build it how he liked anyway, but next time I think I might read one of this book first. (Or if we are lucky enough to have snow - go outside and make a real one!)
Snowmen at Night by Caralyn Buehner