Why are items I enchant weaker than the ones I find? For example, I find items with Fortify Smithing of +12% to +20%, but if I disenchant one to make my own, it's only 8%, even with a Grand Soul and the Enchanter perk. How do I craft items that are as powerful as the ones I find?
It sounds like your Enchanting skill is too low. Under the Enchanting skill tree, the first perk is called Enchanter. As you spend perk points in Enchanter, it will increase the power of your new enchantments. To get your new enchantments to double in strength, you'll first need to get your enchanting up to 80, spending perk points on the way. Once you have Enchanter at 5/5, your new enchantments will be twice as strong. For example, when I first placed Fortify Archery on an item, it would only fortify it by 8%. Now, and I'm only at 4/5 Enchanter (80% stronger enchantments), when I do the same enchant, it fortifies by 32%.
In addition to Enchanter, you can spend perks in other Enchanting skills such as "Insightful Enchanter" which will increase Skill enchantments on armor by an addition 25%. There are others that will boost your fire/frost/storm enchantments, as well as health/magicka/stamina boosting enchantments, by 25%. These do stack with enchanter for a total of 125% stronger enchantments when Enchanter is 5/5 and the necessary perks are acquired.
Don't forget about Fortify Enchanting potions. Even if you're not into Alchemy or unable craft high enough potions of Enchanting, you could always find one in the World or buy one from a Merchant. Drink it before your enchanting session to give you a bit of additional boost to your enchants. Beware they wear out in like 30 seconds or so, even whilst the Enchant interface is on.
In addition to the above, you really want to have the enchanting skill at 100 and in the case of smithing(or any skill related enchant), you need the insightful enchanter perk to make the enchantment as strong as possible(with a grand soul gem). With 100 skill, 5 ranks of enchanter, and the insightful enchanter perk(and again a grand level soul) you should get 25% gear with enchanting alone. With the extra effect perk, you could throw alchemy on the neck, hand and finger parts as well(smithing can go on the chest too, alchemy on the head).