In terms of the process, you can use Everclear as a base, and vodka is fine, too. Just find something that is both strong (high proof) and neutral, and pour it into a mason jar. At this point, you can get creative or you can stick to a recipe. I like to improvise, so I basically just combine things that sound like they'd go well together. If needed, I can always make adjustments later.
Here are the four varieties that I made up this time around. The quantities of ingredients that I added were determined pretty indiscriminately, so I'm not presenting them as recipes; they're more to provide inspiration:
Thai chili and mango
Coconut, crystallized ginger, and cumin
At first the vodka was totally clear, but it changed within a day.
In terms of actual recipes, here's one from about.com
2 cups grain alcohol
8 oz dried orange peel, minced
1 tsp cardamom
1 tsp coriander
1/2 tsp caraway or anise water
3/4 cup granulated sugar
The process is that same as the one I outlined above.
Whether you decide to wing it or dig around online for more precise recipes, you'll have fun making homemade bitters. And they make a fantastic gift- in less than an hour, you can have the equivalent of 12 bottles to give away or keep. You'll also save money- I spent less than $20 on a quantity of ingredients that made over $90 worth of bitters if I paid retail.of bitters began as medicinal tonics, although this application has pretty much fallen by the wayside. Nowadays, it is all about the flavor.