You'll need a couple of things for this. First off, I recommend any apple that works well with cooking. With some apples, the sugar breaks down much too quickly to be used in cooking. The types of apples I will use with this include Rome Beauty, Golden Delicious, Jonagold, Jonathon and Gala. The types I stay away from for this are Red Delicious and Macintosh. You will also need a decent mandolin slicer. A dehydrator will work really well for this procedure but they can be done in the oven as well.
Once you've selected your apples, peel and core the apples. I don't want any apple skin or seeds in the mixture; just the fruit. After you've done this, I slice the apples at a maximum of 1/8th inch or preferably, 1/16th inch. Once you've sliced the apples, place them in the dehydrator or oven at 135F (if your oven won't go this low, place at the lowest setting and keep the door cracked open a bit). Check them periodically to test their doneness. In the dehydrator, they took about 6 hours to get to where they were dehydrated enough that they would be able to be ground into a powder. In the oven, it may take a bit longer if there is no airflow.
Now that you've gotten the apple slices dehydrated enough, I simply place them in a container (with no lid) and rest for a day or two in the refrigerator. After a day or so, I like to break up the pieces by simply crushing them with my fingers and then throw them into the burr grinder. The apples will come out as a pretty fine powder. You will probably notice a little bit of stickiness in the powder from the sugars of the apple. At this point, I like to add 2% of silicon dioxide (an anti-caking agent) to the powder to keep it flowing pretty freely. I also store this powder in the refrigerator as I feel it helps with the stickiness from the sugars.
Your powder should look something like this when finished:
The clumps of powder break up easily with a little pressure from your fingers or a fork.
That's really all there is to it. Some words of advice when using this powder. If you're planning on using this as part of a rub, it is easy for the apple to get overpowered by other spices. Either make this the largest portion of your rub or compliment it with other spices that are not overpowering. You can also use this as a dusting after the meat or vegetables are finished. You also might want to try adding some brown sugar or a minimal amount of cinnamon when dehydrating the apples. This will add another layer of flavor to compliment the apple and the dish you're creating. Sometimes, I'll also use some lemon zest to really play up the citrus component of this spice.
They say that variety is the spice of life; I say spice is the variety of life. There are many things out there that lend themselves to being used as spices. We're limited only by our imagination.