Do the same like I did couple of years ago. Comprehend what JoyMax did with the database. The most important tables for adding (in this case) avatars are
_RefObjCommon (regular initial string/part of the item),
_RefObjItem (for the item behavior, stackability, "Blues" slot count, etc.),
_RefPackageOfItem (the initial shop entry for items, having links to / recalls data of the table RefObjCommon. This table does also handle the "Popularity" of an item. If you open your item mall, you will get the welcome page with new, hot and recommended items. You can set them by tweaking the "SaleTag" in this table for that specific item),
_RefPricePolicyOfItem (obviously price stuff, if gold, silk, giftsilk, etc),
_RefScrapOfPackageItem (in this case nothing special, just an entry. For everything else, it might be additional conditions like durability count on purchase, inheriting blues, etc),
and finally,
_RefShopGoods (adding the item to the item mall. The most important part, also a critical one, here is: SlotIndex. If you have for instance 10 avatars at the "MALL_AVATAR_DRESS" (which is the regular Dress tab at your Item Mall ingame), the SlotIndex stops counting at "9". If you add a new avatar to the shop, assure that the SlotIndex is always a "Previous_Count+1", which is in this case "10" for your 11th Avatar Dress).
It is all about searching, viewing, comprehending, learning and executing. Just search for existing avatars and "backengineer" the stuff JoyMax did there. There is no better guide then teaching it yourself the things on your personal taste, speed and way.
If you want to learn Stored Procedure stuff, frankly not a guide but yet fully working stored procedures, look for "WitchyMoo"s (was he/she called like that?) threads. I have read his/her queries and whatnot, and was able to create my own custom advanced things for my database. No one needs to be ashamed about learning stuff from someone else, so, stick to it, advance yourself. Knowledge is everything.
Have a nice day and good luck with your editing and project!