First off, people do not train iguanas. Iguanas train owners. I always hear people ask how to toilet train an iguana. Once the iguana has decided where their personal spot is, just drop the paper on that spot. An owner can help by providing a spot on the bottom of the cage or giving the iguana a bath every day. Many iguanas will not use their cage for a bathroom unless absolutely necessary. Felix is the exception to this. He makes a mess to get baths. His goal is to get more than one bath a day. If he doesn't get his bath in time, he will drag the poop and paint the cage walls with it. See what I mean about iguanas training their owners?
Iguanas are very good at punishing their owners for going on vacation. They usually starve themelves. Iguanas hate change.