@ratvn:  Disclaimer first.  I have never replaced a nut on a violin.  Many other stringed instruments though, I have done it.  Been doing it on guitars since I first broke one in about 1978.  Some people here will probably yell about this, but I'll tell you how I'd go about it with *my* violin if I had to.  I will also mention here that I wouldn't hesitate to go ahead and do it myself if I felt it was needed.  Bear in mind though, I'm pretty sure some folks would think I belong in a loony bin for some of the things I'll do with instrument repair and mods.

First off, ebony nuts are not expensive.  Stew Mac has them for about 99 cents apiece.  So I *would* buy several, just in case.  May as well, if you're paying the shipping for one.  I personally would also buy some extra bridge blanks so I could tinker with those a bit while keeping one that def works that I could go back to.  Other folks might not feel that is necessary, which is fine.  I won't debate it, just sayin' it is what I would do.

To take off the old nut, first take some blue painter's masking tape.  It will protect the wood you want to keep, and it doesn't leave gunk on it, if it's just on for a little while.  First I'd wrap tape around the neck and fingerboard near the nut.  Then I'd tape the pegbox with a couple of wraps.  That is to help discourage the wood from splintering and protect it while you're working.

Then put down some padding, like a thick towel or something to the violin's finish doesn't get messed up while you work on it.  Take a thin knife like an x-acto or a razor blade.  *CAREFULLY* score the glue line where the nut meets the fingerboard, neck and pegbox.  Let me explain "carefully".  Plan your cuts to be away from your fingers and away from any parts of the wood you want to keep pretty.  Accidents happen.  Always expect them to happen and plan for them as well as you can.  Remember you can turn the violin over, move it around, etc to get a good angle to cut from to be cutting away from your fingers and the pretty wood you want to keep.

After you get the glue line scored, lay the violin on it's back and take something like a wood dowel or a flathead screwdriver and lay it on the taped fingerboard so it is against the nut.  Give it a little tap with a small hammer or maybe the handle of another scewdriver if you don't have a small hammer like a tack hammer around.  If the nut doesn't pop right off, then give it a little harder tap.  If that doesn't do it, score it again.  Repeat the process until it does come off.  It will.  But don't try to cut deep any of the times you are scoring or give it a real hard rap.  Be firm but patient with this step, because it is the one most likely for damage or injury to happen while you are doing it.

Ok, once the nut is off, check the wood where it was.  Clean up any remaining glue with your little blade or a bit of sandpaper if necessary.  If a bit of the wood from the pegbox came off with the nut, then you'll need to fix that, if it necessary.  Just a little splinter or so isn't enough to worry about, since the new nut and glue will cover it.

Once that is clean and square, I'd personally pre-shape the new nut.  File or cut away on it until it is shaped nice (look up detail pics of some expensive violin to see how it should be shaped and how it should look) But leave at least a mm more wood on the top than you think you may need.  Keep stopping and checking it by fitting it where it will go on the instrument to see and feel if you are doing it right.  

When you have it down to the shape and size you want, Mix up a little bit of hide glue.  Don't cook it, just let the powder or crystals dissolve in about enough warm tap water to cover them for a half hour or so.  You do NOT want real strong glue on this join because you want it to pop off fairly easy if you need to replace it again later.  Sooner or later, if not you, then someone will have to.  Nuts do not last forever, just a very long time if nothing happens to damage them.   

The nut doesn't need glued down like it has to survive a zombie apocalypse or anything.  The strings will mostly hold it in place almost enough just on their own.  So just a little glue, and mix if cold rather than heating it to get really powerful hide glue. 

Ok, then check the fit one more time, and if you're sure it is how you want it, glue it into place. 

Then I'd use the strings as a "clamp" to hold it in place to dry overnight.

After it has had overnight to dry, then tune up the violin with just letting the strings sit on the nut with no grooves cut yet.  If they were put on the pegs right, they'll sit in at least almost the right spots even without any grooves.  Play it a little and see if it is close enough for you or if you want to file a little more off.  Put some painter's tape on the fingerboard and pegbox again for filing, and cover the violin with at least paper towels, because ebony dust gets all over the instrument pretty easy.  When you have it down to where you like how it feels, hen you are ready to mark and cut the grooves.  If you take it down too far, then go back to the step for removing the nut, take another blank and make a new one.

When you get the height to just about where you like it, measure and mark the grooves for the strings.  If that was good on the old nut you took off, copy it from that.  If not, look them up.  I really don't like deep grooves/slots.  I like them only as deep as they have to be to keep the string in place with hard playing.  So I just mark them with single light cuts with an x-acto or razor and a steel straightedge or ruler.  Now they need shaped a little.  They make files for that, but I've never actually owned a set.  For the plain strings, I use the back of the exacto blade and press it against the wood of those marks.  it only takes a little scratch dent.  For the wound strings, I use a couple inches of an old wound guitar G string that is superglued to the edge of a popsicle stick.  Don't do much.  If you can see the groove, it is probably deep enough.  Put the strings in the grooves and tune up and try plucking the strings or strumming them like a ukelele to see if they come out of the grooves.  If they don't, you're fine.

If the buzz against the fingerboard when you bow hard, you went too low.  Take another blank and start over.

But if it is all good, then you can check for any little bits of glue you may have missed and carefully (see definition of "careful" above) scrape the glue off with your little blade.  

Then loosen the strings just enough to move them slightly out of the grooves and put a little graphite in the grooves.  just coloring the grooves in black with a sharp pencil will do.  

Put the strings back in place, tune her back up, and you're ready to go.  Pat yourself on the back and call yourself a genius and "let the party and the sounds rock on".

I probably left some stuff out of all that or didn't explain some steps so well.  Feel free to ask, if any of it is unclear to you.

Now, I am sure some folks here know better ways, and can spot at least a hundred spots where I was an idiot and gave poor advice.  Cool.  They can type up their thousand or so word version of step by step instructions that are better.  That'll be great.  Honest.  I'm always looking to learn new tricks.

But this was how I would go about it, and I would expect it to work for a violin the same as it has for many instruments over the years I've worked on.  I *did* trim and groove the nut for my old Mendini, and has been fine with stings staying in place and tuning up just fine and nice low action.  It doesn't buzz either.  I will admit I haven't done a full replacement of a violin nut yet.  But I definitely would, as soon as I feel it necessary.