Since magic item creation has a considerable effect on the campaign setting, and is under the DM control, I'm clarifying here my views on item creation, to provide opportunity for comment before people reach a level where they may take item creation feats. I encourage you to discuss your plans for item creation with me ahead of time, so you don't get stuck with a feat that's not useful for your purposes. Note that the prices in the DMG are explicitly *guidelines* and final prices are set by the DM, although I'm open to reason.
NPCs will play by the same rules as PCs in item creation, but there may be artifacts or gifts of the gods that do not.
Since the campaign is on a silver standard, all costs shown in the DMG as gold pieces should be read as silver pieces instead, and the craft skill is similarly adjusted by a factor of 10.
Some magic items will wind up intelligent. If you deliberately want to create an intelligent item, that may be harder, and require a lot of research, but as always, the overarching rule of efficacious blandishment may be employed to make the task easier.
Magic is intensely personal. The Spellcaster must create the item he is enchanting, and must have any spell prerequisites himself, not via scroll or wand or other caster.
Armor & Shields, Weapons, Rings, Staves and Rods must be Masterwork. Items which grant a skill bonus must be masterwork, and the creator must have at least as many ranks in that skill as the bonus granted.
Wands, scrolls & potions are never required to be masterwork, but can be, if you just want to make a nice looking wand, or a really tasty potion.
Major wondrous items (30,000gp and above) have to be masterwork, but lesser wondrous items do not. It's nice if they are though, especially if you're planning on trying to sell them.
Use-activated items are things like swords and armor, where just using or wearing them starts the effect. Use-activated clothing works when you put it on.
For command word activated items, note that activating is a standard action that does not provoke an attack of opportunity.
Potions are only available for spells that target a creature or creatures, and the drinker is both caster and target. Spells with target "personal" cannot be made into potions. You can use scrolls and wands (but not other casters) to place a spell in a potion, it uses up a charge.
To make a magic item (other than scrolls) you need a recipe. I'm still working on rules for that. Power components (DMG 36) may be used. Creating magic items will require the use of special materials, to be discovered in play. Knowledge (Arcana) is very useful for this. Typically, expect to need appropriate components from a creature whose challenge rating is roughly equal to the caster level of the item. So for a potion of Darkvision, you need the eyes of something that has that ability.
Typically, spells that are prereq for Enhancements cannot be built into items themselves, except as scrolls, potions and wands. For example, you can't make an item that casts Bull's Strength or Mage Armor on you, instead you make Gauntlets of Strength or Bracers of Armor, with the cost determined by the square of the bonus. You can't make an item (other than a wand or scroll) that allows you to cast Shield, although you can make brooches of shielding to soak up 101 points of magic missile damage. No magic items provide a dodge bonus (although it may be possible for an item to grant a feat, but the creator would have to have that feat).
Improvements: The creator of an item can "open it up" later and improve it by adding more enchantments. Cost and XP would be equal to the difference between the costs of the existing enchantment and the new enchantment, but time would be equal to the total cost. E.g. You could make a +1 sword (2 days, 1000gp in materials and 80xp) as a 5th level spellcaster and when you reached 6th level improve it to +2 for another 3000gp, 240xp, taking 8 days. Improving magical items made by someone other than yourself would require magical research, and might not be possible at all, depending on the item. Having access to their notes (or themselves) would help.
Items made in the forge of souls in Khundrakar can be improved by someone using the same forge, after sufficient study.