I still don't really understand how reforging works. At first i though you could add to weapons armor that you find/have but from what i can tell, you can only reforge items that have no mods on them (except normal bonuses) and you can only add 2-3 mods before it goes all cursed/brittle/irreparable
The target item has to be GM made and it can't be made with special materials like barbed leather or valorite ingots (dragon armor, unfortunately, won't work). Unlike imbuing, some results of re-forging are random. However, you get to choose, to an extent, whether the item is cursed/brittle/irreparable.
As an example, to turn an item into a -15 Mage Weapon you need a GM crafted weapon made with iron ingots or normal wood. Then you take a decent runic tool and apply Powerful Re-Forging as well as Grand Artifice, then Inspired Artifice, which will allow you to pick the mod "Exquisite / of Quality." Just those three options. If you choose Structural or some of the other ones before Grand Artifice, you'll end up with better mods but a few disadvantages as well.
The better the runic tool, the better your chances of higher mods like -15 Mage (original tables were -29 to -20). Most of the time you'll end up getting Self Repair, or Durability increase, and sometimes you'll get mods that have nothing to do with Exquisite / of Quality, which is the real annoying part. Some re-forge jobs seem easier, for example, applying an Eater value onto armor (Powerful, Grand/Inspired -> Mighty / of Vitality).
Once you have your re-forged item, you can then enhance, imbue, or imbue and then enhance. For the sake of a -15 mage weapon, you really don't have to enhance unless you use wood, in which case, most are using Bloodwood after they imbue in order to add HPR 2 and HLL (and if they aren't using the Origin store tool for 100% chance I'd be surprised). Using ore-based enhancements will not give you anything close to as beneficial.
Re-forging is rather flexible albeit random. You could take a basic item and apply Exquisite / of Quality just for extra resists. You could end up with a metal item, for example, that has over 20 fire resist, which you can then enhance with Valorite (4/0/3/3/3--doesn't add fire). There's a lot of interesting things you can do with re-forging.