Custom Craft

How to use custom craft in explain please ?

 

with ShapedRecipe class

 

PHP:

public function onEnable(){
        $this->getServer()->getPluginManager()->registerEvents($this, $this);
        $i = Item::get(Item::CHAINMAIL_HELMET);
        $helmet = new ShapedRecipe(["AAA","A A","   "],["A"=>Item::get(Item::EMERALD)],[$i]);
        $helmet->registerToCraftingManager($this->getServer()->getCraftingManager());

 

OP asked to explain not to spoon feed

 

Sorry, but
Where is the position of the AAA letters like in the crafting table?