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You either need to get access to the Arcane University (by joining the MG and getting recommendations from the head of the branch in each city), or download the Frostcrag Spire plug-in from Bethesda. Either one gives you access to an "Altar of Enchanting" (in the Chironasium in the AU, or the first floor of FS).
Then you need one Grand Soul Gem filled with a Grand Soul for each piece of armor you want to enchant. Only powerful creatures have Grand Souls, and only Grand Soul Gems will hold them. (Lower-level gems/souls will work, but the bonus will be less than +10). Make sure you have a Grand Soul Gem in your inventory, cast a Soul Trap spell or use an item/weapon with the Soul Trap effect on the target creature, kill it while the Soul Trap effect is still active, and its soul will be trapped in your Soul Gem). Black Soul Gems or the daedric artifact Azura's Star can be used pretty much the same way, but they have their own quirks and involve quests. There are several threads about how to use Soul Gems; search the forum if you need more info.
(If you have empty Grand Soul Gems, there's at least one sure-fire endless source of Grand Souls to fill 'em with. If you become Grand Champion of the arena, you can fight beasts in the arena once per week. If you're high enough level, the beasts will be Minotaur Lords, and they have Grand Souls).
Then just activate the altar, select the item you want to enchant and one of your Grand Soul Gems, pick the Fortify Attribute effect from the list, and choose Strength as the attribute you want to fortify.  | 
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