all fruit have yeast on them, whether domesticated or wild.
i would start by throwing the fruit into a container (like a 1 gallon juice jug), mashing the fruit, then waiting. the yeast on the fruit will wake up in the presence of the sugars and get to work.
according to the links below, the average Brix of blackberry juice is 10.0, so about 1.040. assuming you ferment down to 1.000, that will make your wine 5.25% ABV. if you want a stronger wine, you'll need to add sugars - either by concentrating the juices of your berries, or by adding other sources of sugar (honey, white sugar, juice concentrate, etc). i couldn't find any info on wineberry juice sugar content.
http://www.fruitsmart.com/PDFs/TechnicalForms/SSBrixChart.pdf
http://www.law.cornell.edu/cfr/text/19/151.91
I think it would be interesting to add some honey once fermentation has started so you end up making a wild melomel.
you might want to consider adding pectic enzyme. acid blend and tannin are other typical additions, but i'm getting the impression that you want to keep this "natural" and limit the additions (even if they might result in a better wine...)
final thing to consider adding is some nutrient. berries on their own typically don't have everything that yeast need to grow, then again you're not using a typical yeast. certainly couldn't hurt to add a few nutrients.
