Here


Code:
/script DoCraft(GetCraftSelectionIndex());
/click ContainerFrame0Item1
/script ReplaceEnchant();
The above macro will attempt to enchant the item in the first slot of your default bag. In order to use this macro, you had to keep your enchanting window open and select/highlight the enchantment you wish to work on.

Code:
/script DoCraft(GetCraftSelectionIndex());
/click CharacterFeetSlot
/script ReplaceEnchant();
This one however will enchant your feet slot item, or your boots. You can change CharacterFeetSlot to CharacterWristSlot for bracer. And the rest of inventory slot as following, CharacterChestSlot, CharacterBackSlot, CharacterHandsSlot. These should be pretty self-explainable.

As a side note,
Code:
/script ReplaceEnchant()
also work for overwriting existing poison.