To customize a forged item with a custom appearance, you need to utilize the crafting guilds in game. So, your first step is to stop by Thrummal over by Delos (v42129). Thrummal serves two important purposes.
The first purpose is to train apprenticeships, which come in three flavours - Blades, Bludgeons and Thrown. Apprenticing will consistently improve the quality of your forgings of that type. It costs 10,000g to retrain your apprenticeship.
The second purpose is to sell designs! Much like tailoring or jewelcraft, custom forging designs go through the craft guild queue. There are rules for this (it must be something a mortal could make, usual crafting rules, must be feasible in combat, etc). To begin, DESIGN REQUEST (costs 300g) to recieve a blank design. Then, after reviewing the WARES in Thrummal's office, DESIGN (your draft number) COPY (the item type you wish to make's number).
Fill out the necessary information - appearance (7 words or 40 characters), dropped (max 80 characters), and examined (minimum two 80 wrapwidth lines). If you have ingredients that aren't included in the base commodities, you need to add them at this point. Once it looks the way you want (DESIGN will list all of the available syntaxes), DESIGN (number) SUBMIT at a crafting annex and wait! Once it is approved (DESIGN (num) PAY FEE), you're ready to go.
Once you have a tangible, forged item made, you then can CUSTOMIZE (item) AS DESIGN (your new design!). Keep in mind that customizing will NOT require you to have a second set of the base commodities (those that are used to forge the item). The only additional items you will need are those you added to the design (such as a cloth grip on a warhammer, you would add 1 cloth to the design, and need 1 cloth to customize).
Once I finish the maths I'll fill this in. 'New Forging' has a capped scale for what you can forge, and one that is fairly achievable (refraining from commenting!). As it stands, the higher a stat goes, the less it improves in action (that is to say, going from 100-135 is a much bigger difference than going from 150-180). Math math math.