You learn it by disenchanting an item with the fortify enchantment item which you have to buy or find yourself.
There is no "fortify enchantment" enchantment - if there were, you'd be able to create infinitely powerful enchanting gear by enchanting +enchanting stuff, putting it on and enchanting more powerful +enchanting gear, indefinitely. As it is you can do the same thing, though with a few more steps, by enchanting +alchemy gear, crafting a +enchanting potion, enchanting new +alchemy gear, new potion, etc. This is generally gamebreaking (even with one cycle of this you can enchant +combat damage gear that will make you monstrously overpowered, up around twice the damage increase you could get from combat skill perks.)
*edit - Woops, just reread your quote, you were talking about fortify enchantments in general, not fortify enchantment enchantments. My bad.