I built one from mud a few years ago. Actually there are now special tips, it's kind of wondering by yourself and get it your way with a personal touch. I do recommend to get it right, otherwise you can spend hours in trying to build something that would last after the first shower of the winter- Don't be cheap on mud, place it on a flat platfrom, and make sure that the platform is well balanced.
Here are some more detailed instructions, similar to the one I have.