There are a lot of threads that mention this. The basic drill seems to be to put the chameleon on a plant that is either "safe" (not toxic) or fake and then put that plant in your shower.
Of course, the order of things is a bit weird. Before you put your chameleon in the shower you need to get it to the right temperature...so, bring the chameleon into the bathroom, set it down and work on the shower.
Unless you have a low flow shower head that gives a MIST, you should aim your shower at a wall. Check the temperature of the water coming off the wall. It should feel comfortably warm to you. Not hot. You should be able to keep your hand in it.
Then, put the chameleon and plant in the path of the "bounce back" from where the shower hits the wall...plan on this being a pretty long experience. Possibly 10 minutes or even more. You want your chameleon to decide he needs to drink and some chameleons take longer to make that decision than others. Some will make it quickly, but I suspect those chameleons handle the cage water so well they rarely end up in a shower.
Also, there's a sort of psychotic thing with chameleons and drinking....people will ask you "have you seen him drink?" (it's a question on the form) but then tell you that they never see their chameleons drink because they do that in private...so...I guess don't worry about whether you see the drinking, just worry about the signs of dehydration (sunken eyes, yellow "urate" (that part of the poop that should be white) ).