The main, and to some extent the easiest, way to obtain ores is to simply extract them from a node - doing so, you will get from 3 to 4 ores every time. It is the simplest, yet at the same time the most time-consuming process - to be able to make even the lowest items from Blacksmithing, you will need from several to over a dozen metal bars, meaning that if you want to create a single item, you will have to find and extract ore from at least a few nodes. Extracting ores from nodes (and smelting them into metal bars) is profitable only during the early, lower levels, when you have little to none access to items, which can be used in the deconstruction process.
A lot more efficient method is the mentioned deconstruction. After going through several levels of experience (when the enemies start to drop items made from steel), you will start to collect a simply absurd number of items. Most of them, obviously, will be way weaker than the ones you will have equipped at the time, so you will have to get rid of them, one way or the other.
It is worth doing so in the process of deconstruction, as you will earn insane amounts of experience corresponding to the branch of crafting you are deconstructing items from (which will be further explained later on). By destroying an item, you have a chance to get from 1 to 3 metal bars (according to the quality of the item), as well as a Temper, a component needed in order to craft an item in a certain style, or a material required to make an item with a given Trait.
The process of deconstruction is fairly simple - to get a desired metal bar, you must destroy an item made from that certain metal. If you want to get Steel Bars, you have to destroy items made from Steel, and so on. If you want to get a specific material required to make an item with a certain Trait (for instance, a Sapphire, required in order to make an item enhancing the effects of Mundus Stones), you will have to destroy an item with that Trait.
Below you will find a table representing the places (for all 3 fractions), in which you can find specific ore nodes: