You'll receive the Fun-size Forge not long into the game from Erik. It is used to craft new weapons and armour, as well as improve those you currently have in your inventory.
Crafting takes the form of a minigame which requires you to 'bash' several parts of your work into a green-range indicated on your screen. Bashing at higher temperatures fills the bar more than at lower temperatures (generally).
To craft a +3 equipment piece, you'll want to aim to get one of the bars directly in the centre of the green range, underneath a small diamond. The bar should go gold when this is achieved. Then make sure the other bars are as close as possible to this point.
Appraise will tell you what bonus you will be getting. Pretty well is +1, Really well is +2, and Brilliantly is +3. Obviously, you'll want this to say Brilliant before finishing your item.
This achievement is not too difficult and you don't need to really know any more than above. You should be able to get this easily in 2 or 3 tries with one of the first recipes you are given.