Repairing a crack on the TOP plate of a Guitar/Violin/Cello etc. is a simple procedure that you can do yourself,(unless you wish to fork over the 100's for professional work) given the right conditions. these are as follows.
Warning- Don't attempt this unless you really trust yourself. (or are strapped for cash) Don't even think about it on a valuable instrument. While this is a professional, modern method, reading does not equate to muscle memory, as I'm sure the musicians in the audience already know. Anyhoo...
-First, the instrument must have a spruce or pine top. All violins/cellos/double basses, and most guitars meet this requirement. The wood most commonly used for the top plate of acoustic strings is cut on the quarter. (i.e. looking down on the upright log, it is sliced like a pie, as opposed to slab cut which is cut like a loaf of bread.) This means there should be relatively straight, close lines of grain (tree rings) flowing up and down the top... baby simple.
-Second, it must be real wood, not plywood, laminate, fiber board, etc. The majority of affordable instruments used to be plywood. Nowadays though, decent, real wood instruments are coming out of workshops in still emerging economies like China, and Former Eastern Bloc countries, especially Romania and The Czech Republic. If your instrument lacks these bold, straight grain lines, or the crack does not occur on such a line(some shops use veneer to fool you)
it's probably plywood. Sorry, don't trust the man at the pawn shop next time.
Alrighty, you've got a real spruce top that needs some lovin. Now you'll need a flat, long piece of metal, thin enough to bend, but not too wobbly. Think long metal ruler.
Insert this into the instrument's sound hole or f-hole. When it touches the back plate, mark the rod at the point it sticks out of the instrument, and bend it at the mark into the shape of a check mark or 'V' with a long/short side. Good, Strad would be pleased.
The trough of your metal 'V' should rest on the back plate, inside the instrument, beneath the crack being persecuted. Now, pull back your side of the 'V' to tilt the
other tip of the 'V' (the trough of the v, resting on the bottom, and acting as fulcrum) to push up on the crack from underneath. GENTLY. The wood should pucker up 'round the crack, don't push too hard or you'll enlarge it, just open er up. Hold it like this. Backtrack.
Thanks to our brave men and women overseas, we no longer have to dismantle and carve out the afflicted instrument. This is good, as surgery is just as bad for fiddles and their kin as it is for we folk. Nope, thanks to Uncle Sam we now have Super Glue, which we can drip a thin line of into the puckered crack, along its entire length, making sure to get plenty on either end so as to prevent it spreading in the future. Just be careful not to get any on the outside plate, ruining the varnish. This completed, quickly press the crack back together, making sure it's perfectly matched, level, and in its natural, pre-crack configuration, or you'll have done more harm than good.
Let the superglue dry. Now, drunk with self satisfaction, proceed to make and apply varnish...fail. Carve and set a neck...fail again. Finally, in an attempt to regain your sense of worth, create a writeup on your only successful endeavor in the wacky world of luthierie.