Okay, so for the next steps.. You have the nut trimmed down more to size, so now it is time to shape it a bit.

You want to round the side that is towards the scroll, but leave the side that is towards the bridge as a nice straight drop-off.  So that looking at it from the G string side, it looks something like this...

 Now, the reason for doing it this way is because if we round the side facing the bridge, it could buzz or give other odd sounds.  So we want that transition for the string to be as clean and sudden as possible.  We round the side that is towards the pegs so the drop-off is more gradual, to avoid making another stress-spot that the string is likely to break at.  

So you take your file and put on another CD to listen to and take your time to get the side towards the scroll sloping down in a nice gentle curve.  Exactly what curve?  Well, there probably is some exact curve that actual luthiers follow, but on a beginner instrument it should be fine if it is just rounded.  But if you want to go for the extra point, hit the internet and look for some side view pics of really nice expensive violins played by great players, where the resolution is good enough that you can zoom in and see the curve.

Which brings us to the next step.  Search the internet and find a good high resolution pic of a really good violin, where it is facing the camera straight on (so you see the top and fingerboard and etc, not the sides).  Ideally, it should be a pic of a really good violin, maybe even one that one of your heroes plays on.  But in any case, a real good one, where you can be sure that the luthier who did the work on it did a great job.

Then take a scrap of paper and take the tape off your fingerboard and lay the paper on it and mark exactly where the sides of your fingerboard are where they meet the nut.  In other words, the width of the fingerboard.  

Now take that pretty pic of a violin that a professional did and open it in your usual photo editing software and adjust the size/zoom until the width of the fingerboard in the pic exactly matches the width you marked on the paper.  Holding the paper up against the computer screen, use your pencil on the paper to mark the center of where each string is on the professionally done nut. 

There is a more proper way of doing this, with measuring and using a set of dividers and etc, but unless you have good measuring tools and the dividers and so on, and some experience in using them, eyes and a pencil and paper are easier for most folks.  Besides, authorities do not all agree on the exact measurements for the string spacing, and copying what you can from instruments that definitely work well is usually more reliable than trying to match what some book says is perfect.  At least in my opinion.

Get the marks as perfect as you can, remembering to use the eraser on the pencil if necessary, to do it as many times as you need to to get it as good as you feel is possible.

Now you take the paper and use it as a guide to draw pencil marks on the top of the nut, again, erasing and re-doing if necessary, until it is as good as you can make it.

Now it is finally time to use that x-acto or razor blade or other thin sharp knife or blade.  Looking very close, lay the sharp edge of the blade right along the center of one of the pencil marks you made on the top of the nut.  Put your thumb against the back of the blade (the not sharp side) and press firmly.  Don't slide the blade, to cut.. just press.   Then check with your fingernail or thumbnail to see if it made enough of a groove or dent that you can feel.  If not, do it again, pressing a bit harder.

Repeat that for all 4 marks you made for the string grooves on the nut. 

Now for some good news.  For your E string slot, you are probably already done.  String grooves should be about 1/3 of the diameter of the string.  Most violin E strings are around .010 inches in diameter, so the groove for it only needs to be about .003 inches.  That is super tiny, and if you can feel a groove there with your fingernail, it is probably plenty deep enough. 

The others will need widened and deepened a little, though.  So you take your nail file, and lay is so the edge between the abrasive side of the file and the side of the file is lying in the groove.  Give it a couple of strokes, then turn the file around so you also file the other side of the groove.  Again just a couple strokes.  That will probably be deep enough for the A string.  A strings aren't very thick either. 

You don't want to make the grooves any deeper than you absolutely have to.  Deep grooves tend to make the note played on the open strings have a more harsh/tinny sound.  They also can make the string stick too much when you are tuning so the string is more likely to break.  You can always take more off later, if you have any problems with the string not staying in place when you play.

The D and G string will take more file work.  But still not much, compared to any of the work you've already done.  If you are not sure if it is deep enough, leave it and put the strings back in place and tune it up and see how it works.  If it seems to work fine, then it probably is deep enough.

Some other folks may point out here that using a nail file like this will give you a v-shaped groove, rather than a round one.  First off, v-shaped isn't all that bad, and the string will round it within a week or two anyway.  Secondly, I have never yet met a nail file that actually has an edge that square and perfect, so it will be at least somewhat rounded.  It will work well enough, but hey.. If you happen to have a set of specialized round files that small, or an abrasive cord, or even an old wound string, you can make the bottom more rounded if you like.

Right before putting the strings in the grooves, it is good to use the sharp blade to scrape a little of the graphite "pencil lead" off and into the grooves.  It acts as a lubricant to make tuning easier, more consistent and less likely to break strings.

So you put the strings back in place and tune it up and basically you're done. 

You might want to file down the edges of the nut along the sides, where your hand might touch it, to make them rounded and smooth.  That isn't a part of the nut that touches the string, so it won't mess anything up whether you leave it as it is or round it a bit.  Some players count on an edge they can feel there, others like it smooth.  So call that optional and up to your personal tastes. 

So there ya go, Dino.  That's at least one way it can be done, with minimal tools.  It will get the string height in the nut down to at least an acceptable place where it shouldn't interfere with comfort or intonation.  You may be able to actually go even a bit lower, depending on the type of strings you use, the bridge height, and how straight your fingerboard is.  But for basic beginner purposes, the thickness of a credit/bank/gift card will usually work pretty well. 

Hope it helps.

Well, Tucson.. If there's too many words in my posts for your liking, feel free to just skip them.  It's a free forum, and not like there'll be a quiz on Friday or anything.  LOL