Well for the visor, assuming you get an acrylic one, you can either use a blow dryer or dip it in boiling water. I tried the boiling water, and it took forever. I started with the blowdryer, I rested the visor in place, and just went over it with the dryer. Granted, it took along time, but was easier to shape because it was already on there while heated, instead of dipping it and laying it down, dipping, etc.
Just when your using a dryer, wear some thick gloves, it will be very hot.
Also, the plastic that the helmet is made of will become very flexible. If you want to get the "flare" that it lacks, some people use a sturdy metal half-loop in the inner bottom of the back of the helmet to make it rounder, instead of the natural oval shape its made like.
For a good visor, the one I have and alot of members here use, get it from BobaMaker.co.uk
-tubachris