Life Size Clown Instruction?
Try here http://www.halloweenmonsterlist.info/ or here http://www.hauntproject.com/
Many of the how-tos are fairly simple with inexpensive products. The head will be the difficult part if you have never done one before. Maybe find a cheap generic mask in a store and customize it with your own paint and a big nose.
Go to Walmart or another discount store and check the remnants bins in the fabric department for cheap material to make the costume. If you aren't very good at sewing or patterns get some old clothes and cut the material to fit over the pants/shirt and sew it onto them. Simply stuffing the old clothes is also an easy way to form the body.