I posted up pics and instructions on how to make your own template and T-visor in another thread. I can't remember which one, but I'll re-post it here later on for you.
Here's the visor that I used for three helmets that I've scratch-built. It's a dark green replaceable safety face shield and it cost me under $8 Canadian after all taxes. With a dark background behind it, it appears black.
Once you've got a face shield, you'll have to make a template to aid in both cutting and mounting the finished t-visor. Take some cardboard ( from a cereal box works great ) and roughly cut it so that you can fit it on the inside of the visor opening... remove just enough material so that it'll clear the insides of the cheeks and sit relatively flush. ( I keep the top edge against the inside ridge that's just below the dome, this way I won't have to cut that edge on my template and ensures a snug fit that'll make it a snap to line up the finished visor. )
Hold or loosely tape the cardboard in place, as you'll need to remove it later. Take a pencil, and from the outside, trace the opening onto the cardboard. Now you can remove the cardboard and discard the tape and draw your template. ( I like to give myself a 2cm edge around the visor opening with extra at the ears. ) Once you've drawn your lines, you can now plot your mounting hole placement. ( These holes in your template will be used to mark locations on both the t-visor and on the inside of the helmet to ensure that they line up. )
Your template should look something like this...
To transfer the template onto the face shield, I first taped off the entire front surface of the shield with green painters tape. Mark the centre of the visor and tape the template in place. Trace onto the tape remembering the mounting holes, and remove the template. Now it's time to drill the holes, then you can cut out the visor. ( I used a dremel with a reinfoced cut-off disk, works great. ) Clean up the edges with some sand paper or a sanding drum on the dremel. You can remove the tape now, but I prefer to leave it on until after the mounting screws are installed as this prevents any accidental damage to the visor.
When you're done with the template,
don't discard it. I keep mine to use as temporary visors while I work on and paint the helmet.
I used chicago or boston screws with plastic washers used for kitchen faucets, to mount my visors. Just mark the spot and super glue the portion with the inside threads, to the inside of the helmet. This allows for easy removal of the t-visor for either replacement, or to prevent damage while you work on or repair the helmet.
And viola... the finished product!!