It really couldn't be easier. There's a few basic ways of doing it, none of them particularly difficult unless you're using a touchy flavorant.
These are the methods ranked in GENERAL (there are some exceptions for some flavorants) order of intensity of flavor imparted per unit of flavorant used:
- Passive infusion: Place the flavoring agent(s)(or an extract or distillate of the flavorants) in a sealed container with the salt itself. You can put one or the other in a sachet to avoid mixing.. or to maximize flavor transfer, maximize the surface area, spreading them out in a thin layer each, separated by cheesecloth. Allow the natural diffusion of volatiles in the flavorant to gently infuse the salt. If using fresh herbs or spices, it's more effective to crush them, or grind them (and in the case of hard seedpod based spices, toasting them) before the passive infusion.
- Air infusion: Same as the above, but instead of passively letting the flavors float out naturally into a sealed container, you layer the salt and flavorant in cheese cloth (including a cover layer for both... so at least three sheets of cheesecloth), and blow air over or through them (perhaps using a food dehydrator or a furnace filter and box fan), pushing the flavors out of the flavorant, and through the salt. Hot, moist air will be more effective at infusing most flavors into salt, and can be rigged pretty easily depending on the method used. A convection oven set to 150 degrees with a pan of water in the bottom would work just fine. If using dry air, you can start with fresh flavorants (particularly fresh herbs and spices), and as they dry out, you can then grind them up and mix them into the salt (presuming the flavorants aren't too bitter, grainy, woody etc... when eaten thus).
- Vapor infuse the flavor: use a smoker, a vaporizer of some kind (rotovap, steam vaporizer, teakettle etc...), or if the flavors are susceptible to vaporization with low heat without extra water, use a pot and a hot plate etc. Pass the vapor through the salt in a cheesecloth bag (or something similar). You can also create a flavor extract or distillate out of your flavorant, and vapor infuse with that
- Mix method: finely grind, shred, mince, cut, or otherwise powder the flavorant(s), and mix them into the salt directly until evenly distributed (I do not consider this to be a flavored salt so much as a spice mix, but it still works just fine to get the intended effect... at least for most flavorants)
- Slurry method: Makle a slurry with the flavorant (or an extract or distillate of the flavorant), the salt, and a liquid that the salt AND the flavorant are both soluble in (I recommend a mixture of vodka and water. The mix will liberate more flavor from the flavorant than water alone, and evaporate faster). You can also use a flavorful liquid in and of itself, to add further flavor to the salt (or make a flavorful liquid infusion out of a dry flavorant). Use just enough liquid to create an evenly mixed slurry, then spread the slurry out on a silpat, baking sheet lined with parchment paper, dehydrator rack etc. Dry out the slurry, and regrind the salt to the desired texture
- Hot water infusion aka the "Stewing" method: Simmer the salt and your flavorant together in an unsaturated solution with a liquid they are both soluble in (again, I recommend vodka and water). This will liberate the most possible flavor from most flavorants; however some flavorants don't tolerate heat well, as boiling will drive off the volatiles in them. Boil the solution down into a slurry, then handle as above
The pretentious gourmet types seem to like truffle salt (actually, I do too; but only if it's subtly done. Most "truffle salt" tastes funky, in a bad way, not in a good "bleu cheese" kind of way); which is actually ridiculously easy to make by finely shredding the truffle with a microplaner, mixing it into the salt (about 1/4 ounce of truffle to 4 ounces of salt), and allowing it to passively infuse for a few weeks in a sealed jar.
Mel and I make our own smoked salt in our big smoker, using one pound loose weave bags of salt. Smoked salt is really great in soups, and spectacular on popcorn or french fries. I also recommend trying rosemary, thyme, and garlic together as a flavor combination (it will work with all the above, and each method is a little different).
...And of course, there's always Bacon Salt.