Homestead Arcana

The 'recipe' for the Steel Guitar can be purchased from Abraham's store under the 'Upgrades' section. I left all the optional upgrades until after I had unlocked the Money Bags achievement so I'm not sure exactly when it becomes available** and how much it costs (although I can tell you that 20,000 coins are enough to buy all the non-essential upgrades all at once).

** Thanks to CiphriusKane who clarified:
The recipe is available after clearing Pearl Canyon for 375 silver

It is constructed at the Workbench, under the Homestead Improvements section, costs 150 coins and requires:

• Metal Cluster x 2

• Hemp x 2

• Truffle x 2

So, I believe you will have to proceeded into the Swamp area to be able to construct one.

Once constructed, it takes the form of a placable object and looks more like a harpsicord. Allegedly, placing it near your garden will increase the yield, but I never tested it. Simply place it like you would any other object, then interact with it to play and unlock this achievement.

A Steel Guitar also appears in at least one of the Objectives from town.