Know how to bet on chickens, and a prerequisite would be having tons of wooden tickets already from doing so. Here's devilleon's guide on how that works (don't worry, this is very quick 'n' easy!):

So to start, as I've mentioned above - earn about 160,000+ Wooden Tickets from Chicken Betting, and turn that into any amount of Silver Hammers [thanks @Sternutator] to sell at the pawn shop. (you can keep extra for actual crafting later, I guess.) - [You can earn more if you'd like, but you need to leave about 60-70k of extra tickets for later.]

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After going through the process of selling your platinum plates, proceed to figure out the rank you'd want to get the Blacksmith to, we'll use an empty bar of Blacksmith Rank 10 as an example, and our goal will be Blacksmith Rank 15.

A legendary weapon, when donated at rank 10, fills up about less than 33.33% of the bar.

Calculate how big the increase will be on your current level. The donation's increase to the bar scales with each 5 levels of the Blacksmith. Crafting something fills up about 1%. Worry only about the weapons' increase, and figure out if you'd need an extra later.

But anyway, for our example, we would need about 16-17 legendary weapons in order to fully fill up an Rank 10 empty bar up to Rank 15.

In our example, we'll need 17 legendary weapons (just to be sure) to fill up the Level 10 bar up to Level 15. Go back to the Gambling Den in Rakugai, and exchange points to buy Silver Hammers exactly twice the amount of weapons needed.

(17 x 2 = 34 Silver Hammers)

Go to the arms dealer near the Blacksmith's store, and buy the required amount of Bundle Blades (again, for example, 34). Proceed to the blacksmith, and buy about 100 of each material needed to craft the two following swords that stem from the Bundle Blade crafting tree.

[If you ever run out of materials, just buy another stack more; you have money.]

Materials:

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Once you're finished with all of the above, continuously craft Keen Blades using Silver Hammers until you've reached the required amount, then proceed to continuously craft Stone Carvers with Silver Hammers until you've reached the required amount you desire.

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EDIT: It has come to my attention that the bar upgrade scales depending on your current rank. At rank 10, it'd be less than 33.33% of an increase, but at Rank 15, it may only be this low. I've edited the text above to reflect this.