You’ll trigger this quest on the 17th of Winter, via your mailbox. Clint sends a letter asking you to make an Iron Bar for him so he can try out a new hammer. (I’m assuming this one is triggered only if you have a Furnace, as well.)
Unless you luck out and get an Iron Bar in some other way, such as having a monster drop one - I’ve seen a Copper Bar dropped, so I assume an Iron Bar can be dropped too - you’ll have to make it on your own, via a Furnace. You’ll need the following to make your Iron Bar:
- One piece of Coal. Coal is found by busting up rocks, of which there are many on your farm. You can also find it in bags and mine carts throughout the Mines, if you’ve gone deep enough. Barring that you can also make Coal in Charcoal Kilns, assuming you have 20 Wood.
- Five pieces of Iron Ore. You can purchase the Ore from Clint, which seems a little… sketchy… or you can get it by delving into the Mines. Higher-level Geodes occasionally contain Iron Ore, and you can mine it from rocks with silvery protrusions in the Mines, once you get down to about level forty.
Toss your ingredients into a Furnace and wait for a while. Iron Bars typically take a few real-time minutes, so you might be better off setting the Furnace to work overnight and snagging the Iron Bar in the morning.
Take your Iron Bar to Clint. He’s pretty predictable - if he’s not hanging out at the Blacksmith shop during office hours, he’s usually at the Stardust Saloon. He’ll give you 500 gold via your Journal for the Iron Bar.