My African folding chair cracked lengthwise.
I fixed it with epoxy thickened with wood dust. That's usually the best thing, but use whatever you've got. Even Elmer's school glue is okay if the crack isn't separated too far.
If you're fixing a valuable musical instrument talk to a luthier about glues. They'll suck you into a glue-filled time-vortex.
Step 1: Clean the Injury
I pried the crack apart and sanded as high in the crack as I could. Then I pushed the crack together to sand the outer part.
Step 2: Glue in the Joint
Get the metering pumps "EPX-P41" that go with the jugs.
Follow the directions!
After that I mixed in an equal quantity of wood dust from my beltsander to thicken it.
I was doing some boat work and already had some mixed up. By the time I got to the chair the glue was getting thick. I used a pencil to wedge the crack apart. I whittled a wooden lath into a chisel shape and used that to trowel glue into the joint.
Step 3: Work it In
Step 4: Clamp It
I'm using bar clamps because I have them. If you don't have clamps use innertubes, sticks, wedges, or weights to do it.