In the code below i'm making it so a milk bucket acts as a normal bucket. But when "event.entityPlayer.inventory.consumeInventoryItem(Items.milk_bucket);" is called it will consume any milk bucket on the hotbar (in order from left to right) instead of the one in hand.

 

@SubscribeEvent
public void onPlayerInteraction(PlayerInteractEvent event) {
	if(event.action == PlayerInteractEvent.Action.RIGHT_CLICK_BLOCK) {
		if(event.entityPlayer.getHeldItem().getItem() != null && event.entityPlayer.getHeldItem().getItem() == Items.milk_bucket) {
			if(!event.entityPlayer.capabilities.isCreativeMode) {
				event.entityPlayer.inventory.consumeInventoryItem(Items.milk_bucket);
				event.entityPlayer.inventory.addItemStackToInventory(new ItemStack(Items.bucket, 1));
			}
			switch (event.face) {
			case 0: event.world.setBlock(event.x, event.y-1, event.z, FancyCheeses.block_cow_milk); //y-1
			case 1: event.world.setBlock(event.x, event.y+1, event.z, FancyCheeses.block_cow_milk); //y+1
			case 2: event.world.setBlock(event.x, event.y, event.z-1, FancyCheeses.block_cow_milk); //z-1
			case 3: event.world.setBlock(event.x, event.y, event.z+1, FancyCheeses.block_cow_milk); //z+1
			case 4: event.world.setBlock(event.x-1, event.y, event.z, FancyCheeses.block_cow_milk); //x-1
			case 5: event.world.setBlock(event.x+1, event.y, event.z, FancyCheeses.block_cow_milk); //x+1
			}
		}
	}
}

You have to do it yourself, you get the currently selected slot (0- with player.inventory.currentItem.

And then how would I consume the item? ConsumeItem doesn't take a slot.

You can also use player.getHeldItem() to get the currently held item, reduce the stack size as diesieben mentioned, and if it's null, use player.setCurrentItemOrArmor(0, null), because that always sets the currently held item to whatever the second parameter is.

 

Or you could play with player.inventory.currentItem, but I find the above more expedient