Grab one of these off amazon;
Materials needed to get a good visor;
Scissors or a rotary tool with a cutting blade
Xacto knife
Squeegee or credit card to remove bubbles
Water and soap mixture, in a spray bottle is best
Non adhesive window tint
Chicago screws
2 part fast cure epoxy resin
Rubber washers
A drill with a bit wide enough for the Chicago screws to fit through
Industrial strength Velcro
80 grit sandpaper or similar
To start, cut the to shape to fit the interior of your helmet. Start big and then cut little by little to get it to fit as you want it to. To add a curve, boil some water and gently pour it over the shield once you've got it cut, th3n bend and hold in shape. It won't add a big curve, and may cause some slight fogging, but you won't notice while wearing the helmet.
Next, fit your visor into place inside your helmet and trace along the outside so you have an exact set on where the visor will sit in your helmet. In the spot right between your lips and nose, make 2 dots with a marker to mark where your Chicago screws will be glued into place. Sand those spots well with low grit sandpaper, and reapply the dot, then sand the bottom of the Chicago screws. This will give both spots ridges for the glue to hold and bond well. Then mark the same spots on your visor and drill out the holes for the screws. Make sure the bit is just slightly bigger than the screw diameter so it has a tiny bit of room to move.
Once the holes are drilled out, apply the window film following it's instructions. Use the xacto to cut out the film over the holes for your screws, and to cut the film to the exact shape of your visor.
Next up, take the epoxy, mix well and dip the end of the Chicago screws into it. Then place them over the dots in your helmet and hold in place. Let it cure for 24 hours, and you should be good to install your visor. Install through the screw holes, then put the rubber washers on, the screw the screws into place overtop them. Don't go as tight as you can, as you may snap the connection between the glue and the helmet, though if it is fully cured you should be ok.
Once installed, use the soft side of the velcro to make the visor flush to the inside of your helmet, and you are all set. This way you can replace the visor in future, while having a strong and stable install.
Here are images of my helmet and visor application;