I don't have them, but regarding the "how", when you're on Comixology you can "add to cart" or "send as gift" (I don't remember the exact words, and the site is down so I can't confirm). When you have items in your cart that are marked "gift", then before checkout you're brought to a screen that asks for your username, the username of the recipient, and the recipient's email address. You can put a little message if you like. When you pay, the recipient gets an email (possibly to their junk folder, so warn them it was sent), stating that you bought them a gift. They click a hyperlink, log in, and the gifts are in their account.
So when you make an arrangement with someone, just get their Comixology username and email address.
For amounts over $10, you can send eGift cards, which works the same way (top right corner of the homepage). But it only works for $10 or more. I only use them when trading comics in larger quantities.