To make the wild yeast starter, combine the elderberries, sugar, and water in a quart sized jar covered with a tea towel and rubber band, or a fermenting airlock system.
Stir vigorously three times per day, and within 4-5 days you should start to see bubbles forming. Strain out the elderberries when you are ready to make the soda.
Elderberry Soda
Once your starter is going, it's time to make the soda. Combine the water, elderberries, ginger, and cinnamon stick in a large pot and bring to a boil. Turn the heat down to medium and simmer for 20 minutes. Turn off the heat and let the mixture come to room temperature.
Strain the elderberry/ginger mixture into a wide mouth gallon jar and stir in the honey.
Add the strained elderberry starter (or ginger bug) to the gallon jar, then cover the jar with a cheesecloth and rubber band. Put in a cool and dark place to ferment, stirring vigorously daily.
You should start to see active fermentation in the jug after only a day or two. Let it ferment for 5-7 days, then transfer into flip top bottles.
Let the bottles sit at room temperature for a day or two to build up carbonation, then refrigerate.
Notes
If you would rather use a ginger bug, you can skip making the wild elderberry starter and use about one cup of active ginger bug when making the soda instead.
Be extra careful not to bottle too soon so that you don't get exploding bottles. If fermentation hasn't died down after a week I would wait a little bit longer before you transfer to bottles.