I feel like a fool for asking these most basic enchanting questions, BUT...

1. Can you explain how Soul Gems work in Enchanting? Why is it that not all of the my soul gems appear when I am trying to enchant something?

2. What are the factors that determine the quality of the enchant that I put on an item. I see the enchant says adds 6% to Archery, but when I apply it to an item (gloves for instance) it only says 1% increase to Archery.

3. I just joined the Thieves Guild and have been given their armor. I love the look of it, but I want to enchant it with +Archery stats. Can you enchant an item that is already enchanted (pickpocket, carrying capacity, etc). What are my options if I want that same looking armor but with my own stats on it?

I really appreciate the time takes to answer these.