If you've managed to get through the last couple of Hardware Schools on Nut Materials, well done. Hope your stamina holds out to take a look at fitting a nut.
Materials are only part of the picture. For a nut to do a good job, it must be properly fitted and well slotted.
A Good Tight Nut Fit
Proper fitting is really important. Apart from the aesthetics and feel, a nut must make good, solid contact with the neck and fingerboard if it's to work well.
You don't want the nut to impede a string's vibration and a less-than-solid contact between nut and neck is a recipe for sucking out string energy. In the same way that soft plastic nuts can soak up vibration, a badly fitted nut can also kill your tone.
You're looking for a solid, even contact along the bottom of the nut and along the front surface (that butts against the fingerboard). In an ideal world, we don't want to see shims under nuts and any softer shims are definitely out. It's worth mentioning that I occasionally use bone shims. While the first choice would be to replace the nut, a bone shim can do a decent job if that's not an option.
Lots to consider with slotting:
Nut Slots: Spacing
Each string slot in a nut must first be properly spaced. Generally, this spacing isn't the same across the fingerboard but increases very slightly as it moves from 1st string to the bottom—as the strings' get thicker, the spacing gets bigger. We're not talking about very much of a difference but you'd be surprised at how unpleasant things feel if the spacing is incorrect.
Nut Slots: Height
After spacing, the next job is string height. Each string-slot in the nut must be correctly cut to give the optimal setting for that string. If the slot is too low, the string will probably buzz off the first fret when it's played. If the slot is too high, playability will suffer, as will intonation. A too-high slot means you'll need to stretch the string farther when fretting, especially in the lower positions. This can definitely kick out your intonation and make for nastiness.
Nut Slots: Size
Finally, the string slots need to be the right size for each string. I have a lot of gauged nut-files. Each is a slightly different size. I can pick the right file for each string-slot, matching it to the string that it will hold.
Too big a slot—say a .042" slot holding a .016" string—will allow a string some sideways movement which isn't ideal. Worse, however, is too small a slot. This can bind and pinch the string, often causing tuning issues. If you've ever had a string 'ping' as you tuned it, you might have too tight a slot (or the string has begun to wear the slot, basically cutting its only slot and catching in that).
Nut Slots: Depth
You might think this is the same thing as height but there's an extra component to think about. After the slots are cut, the excess nut material above the slots should be removed. While there's an argument that this is (at least partly) for reasons of tone, in my view, it's mostly because it's ugly. Big deep slots and a quarter-inch of nut above the strings just looks a bit rubbish.
Ideally, your wound strings should have about half to two-thirds of their diameter in their slots and the unwound strings should sit just below the surface.
Pre-Cut and Pre-Slotted Nuts
For the most part, any nut I make (whether bone or one of the alternatives) is cut from a blank. This is just a hunk of that material that I cut to size and shape before slotting. This allows me to get the best fit and look for my nuts.
However, many nuts are available as pre-cut and pre-slotted pieces. Personally, I don't really like these but I have the luxury of having the correct tools and a bit of experience. I realise that pre-cut nuts from Tusq, or similar material, can still be an excellent home-fit upgrade from a guitar with a cheapie plastic nut.
If you're doing one yourself, remember that it's not usually a case of just dropping it in. They'll usually require a little sanding and shaping to fit properly. In the absence of gauged nut-files, height adjustment on these is done by removing material from the bottom. This can make for an awkward balancing act to get things right across the bass and treble sides of your neck so go slooooooooow. It's a pain, but do a tiny bit at a time and keep putting the nut back on and retuning (always check when tuned to pitch) to see where your string height is.