How I can create a custom firmware ?

Hello Guys,

So for my iPhone 5 i'm nostalgic and I want to downgrade to the iOS 6.X.X version but is not possible so I've got an idea ! Please how I can create a custom firmware ? (I transfer icons, the design and the wallpapers of the iOS in the .ipsw of the iOS 9.3.3, roughly I have the complete design of the iOS 6 while being under the iOS 9.3.3!)


Thanks for reply