I made a very simple acoustic dulcimer from one piece of wood. This is a very basic version of this type of instrument, how elaborate you make yours depends on your skill level and what tools you have to hand. It's a wonderful instrument that's fun to play and easy to get a good tune out of!

You will need:

A chunky plank of wood

A Saw (handsaw, chopsaw, bandsaw, scrollsaw, whatever you're comfortable with)

Some musical wire (I have a guide on DIY musical wire here: https://vulpestruments.com/strings/)

3 Tuning pins. (I also have a guide on DIY tuning machines if you don't want to purchase zither or piano pins https://vulpestruments.com/tuning-pins/)

2 thick wooden dowels and a few thinner wooden dowels.

Step 1: Hollowing Your Plank

Acoustic instruments need hollow bodies to resonate and amplify vibrations, so our first step is to give our chunky plank a hollow bit.

These steps line up with the images attached.

Start with a nice chunky bit of timber.
Trim a piece off the end. Roughly 3 cm would be a good size, just as long as it's not too thin.
Trim a thin piece off the top, the thinner the better as this will be your soundboard.
This was much thicker than I was thinking of getting, but it still worked nicely.
Hollow out the plank by any means necessary. I used a chisel and a hammer, you could use a router or drill lots of holes then chisel out the bits. It doesn't have to be perfect, but the more material you can get out, the better.
Place and glue the top back on:
And the end bit back on the end. It should now look almost the same as when you started!

Step 2: Add Some Soundholes

You could add some nice decorative f-holes like traditional instruments, or make a bigger hole with a decorative rosette, or just drill one small hole and leave it like that. These holes allow the soundwaves that are bouncing around inside your hollow part to escape and enter the big wide world.

For a basic instrument like this, it's not going to make a huge difference what type of hole you make. Once you get more skilled or start making very fine, detailed and professional grade instruments, the sound holes become more important and more elaborate.

Step 3: Strings and Bridges and Nuts and Frets

The next step is to add the musical parts to your hollow plank with a hole in it.

Add a few eyelets top and bottom of your instrument, these will hold your strings in place. Keep them spaced out and even so that your strings end up parallel to each other.
Add tuning pins. I used piano pins as I happened to have spares. I have a whole tutorial on DIY tuning pins and how to use them on my page here: https://vulpestruments.com/tuning-pins/
Strings get tied to the eyelets at one end, get threaded through the eyelets at the other end and then strung up to whatever tuning method you decide to use. You can use proper musical wire like guitar strings or banjo strings, or you can use stripped down washing line or strong enough steel wire. (https://vulpestruments.com/strings/ for more ideas).
Use the thicker dowels at each end of the instrument to act as your nut and bridge. Make sure you position the bridge part not too close to the edge of the hollow part of your instrument. The closer to the middle of the hollow body the better, but if it makes your strings too short then just move it more towards the end.
Tune your strings to fifths, like ADA or DAD or GCG. These tend to be what dulcimers usually get tuned to and are generally nice sounding tuning systems.

Frets are a complicated part of these instruments, and there are very complex bits of maths that you can do in order to get the exact right placement of the fret with the right ratios between the height of the nut compared to the fretboard and measure to tenths of a millimeter to get it perfect. BUT, we will leave that to the professionals. The easiest way to do DIY frets is a method I figured out using pre-tuned strings and trial and error. If you watch the video attached or check my page on frets here: https://vulpestruments.com/frets/ then it'll make sense.