It's the same experience as making sounds for your game with the only difference that not your hearing but your vision changes. It's best to go out
with your phone or camera. But even in your own home you can discover your required textures.
After that, I started editing my photos in ArtRage
to make nice textures out of it, so I could import them into my game.
And when I didn't have a photo, I made a digital painting myself and used it as a texture.
Just click on the Arrow-buttons