I have used this method hundreds of times (at least 5 times) for bread...I've got to admit that I've been dying to try it on a dark extract and specialty grain and light hop beer...I can't see why it wouldn't work great.
This is borrowed from a web page that I found a few years ago, and I do NOT remember who it is from. Sorry.
Rye flower (Stone Ground) is used to make a new starter because it has the natural yeast and bacteria needed to start the culture. Once your starter is healthy, you can use it with any kind of flour, so you can use rye flour even if you don't plan on making rye bread.
Mix 1/4 cup of chlorine free water (either bottled or filtered will do fine) with 3/8 cup of whole grain flour in a quart sized glass container. Mix them well, cover with plastic wrap or a loose fitting lid, and put in an 85F area. A gas oven with a pilot light, or an oven without a pilot light with it's light bulb on should be close. You might want to put a thermometer in the oven to check it's temperature. Now wait about 12 hours.
There is a very good chance that at the end of the 12 hours, you'll see bubbles in the liquid.
If not, wait another 12 hours. You want to see some bubbles, some signs of life. If you still don't see any signs of life, wait another 12 hours. If you still haven't seen any bubbles or signs of life, ditch the starter and start over.
Once you see signs of life, add another 1/4 cup of chlorine free water and 3/8 cup of whole grain flour. Cover the container, and put it back in the warm area.
At this point, you will need to feed your starter regularly to encourage the growth of the micro-organisms. Every 8 to 12 hours you need to feed the starter. You should double the size of the starter with each feeding. This will get out of control pretty quickly, so to keep from being overwhelmed by starter, you should discard 1/2 the starter before each feeding of the starter.
I have had good luck sticking a jar of this in the fridge and getting it back going a year later.
The trick to this is that EVERYTHING must be 100% steril.