Get this question enough, so here you go... My ODST lens method using replacement motor cycle helmet lenses. Trick being that the lower piece, when cut out, is actually flipped upside down when the the two pieces are put back together. But that is my method, specific to my template and my helmet.
How did I make my template? Well, cheap and easy... I used a 2 liter soda bottle. I cut the bottle and lay the plastic flat on the motor cycle helmet lens, and trace the lens shape onto the bottle plastic. I then, cut this out so I know exactly how much material I have to work with when making my pattern. The pattern is useless if it I can't cut it out of one replacement lens. Once I have a good pattern, I transfer this to the replacement lens, cut the pieces out, mount T-Nuts to the helmet interior. Remember, you need about 1/4 to 1/2 an inch around the edge of the visor so that you have enough room to drill holes in the visor for the t-Nuts to pass through. The lenses are forced to bend to my desired shape and held in place with the t-nuts and machine screws.
So now you know...