The next time you make a Fortify Restoration potion, it should be a little more powerful than the first one. After creating the potion, drink it from the inventory, then unequip and re-equip the four enchanted gear items.
After equipping these items, make a Fortify Restoration potion using a Salt Pile and either Abecian Longfin or Cyrodilic Spadetail, all of which are easily bought in Arcadia's Cauldron. To begin the Restoration Potion glitch, you need to enchant four items with the Fortify Alchemy enchantment: a ring, necklace, gauntlets, and helmet.