Itch is a natural sign and simply looks like you are scratching an itch. Scratch your non-dominant hand with your dominant hand.
You can specify the location of the itch, by scratching where you want to indicate location of the itch. For example, if your neck has an itch, make the scratching motion at you neck.
Itch is a fun sign. When you see your baby scratching themselves, show you understand by doing the sign while saying, "Do you have an itch?"
Itch is a fun and intuitive sign that babies quickly catch on to. When you see them scratching, it is a great time to model the sign and they will quickly understand the meaning.