Blacksmithing is the ability to craft items and materials out of scrap metal.
At first, you won't even be able to use a forge to level up. Scraping items will be the only way to gain Xp. The process is pretty slow, it won't be very effective as you level up, unless you mass-scrap items like nails, that are easily crafted.
The following list will provide scrapable items. Notice cans, iron bolts, nails, makarov parts, revolver parts, spark plugs, and wires do not need a hacksaw for scraping.
Once you can craft bellows and build a forge, the most efficient way to level up is to craft items. Iron pipes are strongly recommended, as they provide 10 Xp per bellows use, and they are needed in multiple crafting recipes.
Appart from the recipes listed above, you can get some blacksmithing xp with the following recipes :