Here's a kind of "starter" bitters recipe that is easy to make and super versatile. This recipe most closely resembles a basic/traditional approach to making aromatic bitters — big baking spices like cinnamon, cloves, and allspice with orange peel and raisins backed up by lots of bittering agents.
If you just use neutral grain spirits as the base alcohol (lose the whiskey, vermouth, and barenjager) and subtract the walnuts, black walnut hulls, hazelnuts, coffee beans, and cut back on the cinnamon, you've got a general "base" recipe.
Combine all of the ingredients except for the water and simple syrup in a large (50-70 ounces) sealable jar.
Shake vigorously once a day for two weeks and taste the mixture intermittently along the way so you can become more sensitive to how the flavor changes. Put a few drops on your hands and rub them together, the cup your nose and mouth and inhale. Others insist on putting a few drops in a glass of soda water or tasting them directly on your tongue, but I've found that the former dilutes the flavor too much and the latter destroys your palette for about 20 minutes.
For the third and fourth week, pay closer attention to your tasting notes — I usually hit gold around three and a half weeks, more than five and you're really almost past the point of no return.
Once you're achieved the desired flavor, strain the mixture through a chinois lined with 5-8 layers of cheesecloth into a nonreactive container. Add mineral water and Turbinado simple syrup to taste. Transfer into a large, cylindrical vessel and let sit for 1-3 days.
Once the sediment has completely fallen to the bottom, either decant or siphon off the clear solution near the top.