First get ML's 3d helmet model and template (
http://www.racedepartment.com/forum...ireframes-for-help-in-skinning-ps-only.66270/ and
http://www.racedepartment.com/forum/threads/helmet-models-for-photoshop.56332/) and make your helmet.
When you're done download this:
https://developer.nvidia.com/nvidia-texture-tools-adobe-photoshop and save your helmet (the 2d texture file, not the 3d helmet model) as a .dds. Just use the default settings.
Then download the pssg editor from here:
http://petar.outer-heaven.net/downloads/ (just get the latest version). If you just want to use your helmet in career mode go to your f1 2012 install directory/characters/bespokehelmets and use the pssg editor to open up any of the helmets there, it doesn't matter which one (make a backup if you may want to use it again later) and import the .dds you saved earlier and save the file. Do this for both the low and high file of the helmet you replaced, you may want to half the resolution for the low file depending on how beefy your computer is. Then just start up the game and in the helmet selection menu select the helmet number of the file you replaced (the menu picture won't have changed) and you should be all set to enjoy your very own helmet ingame.
If you want to use an existing helmet as your base, just do the same in reverse.