For free, there are several books available in most larger libraries as Hollowfox suggested. The easiest one I am aware of is by PHIL MASON......How to Build a Hammered Dulcimer

You don't say what your level of skill is of if you have access to all the parts and pieces incidental to the making of one. Mason's plans build a rectangular (but trapezoidally strung) instrument using simpler techniques and you wind up with a very playable instrument. If your skills are more advanced and you want to use top tonewoods, I'd advise you cough up the dough for a set of plans from either Rick Fogel or Luthier's Mercantile in California. I have seen dulcimers built from these plans and they are very nice instruments.

If your skills and tools are a bit less, I would try a kit. My last thought is that with a nice 12/11 able to be found for $350.00 or less...........Why?

Spaw