How do I make my putter a Belly or Long Putter?

I have read where people have cut off an old iron shaft to extend the length of the putter and making into a belly putter. I'm not so sure that there is a good method for doing this except just buying a belly putter.

There seems to be a difference in construction on a belly putter. Converting a regular putter into a belly putter my not give you the same results as purchasing a belly putter outright.