"armor top" aka Helmets are made by placing the Leather, Gold Ingots, (Fire), Iron Ingots or Diamond Gems in the crafting table in the following way:
|Item|Item|Item|
|Item|none|Item|
|none|none|none|
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iron diamond gold and leather
There is no way to make zombies wear leather armor, but sometimes zombies can naturally spawn already wearing the armor.
Helmet, chest plate, leggings, and boots.
Open the inventory by pressing e, and then put the armor on those 4 slots on the top left. Helmet on top, chestplate below, leggings under that and boots at the bottom.
The best armor in that mod is Halite armor.