Buttermilk is very easy to make; much easier than yogurt.
Like yogurt, you need to start with a culture. I buy buttermilk at the store, then pour 1/4-1/2 cup (I don't measure) into a quart-sized glass jar. Fill the jar the rest of the way up with milk. Stir well. Attach lid to jar. Set buttermilk on counter for 4-24 hours, until the milk has thickened, and smells like buttermilk. The amount of time it takes depends upon the temperature of the room, etc. You can shake or stir it on occasion if you'd like, but it's not necessary. If you leave it too long, it will separate into whey and buttermilk cheese (I think this is called quark). The cheese tastes good!
When you open the jar to use the buttermilk, be sure to take out 1/4-1/2 cup to put in a new jar for your next batch of buttermilk.
I have used powdered milk to successfully make buttermilk, but it did not act well as a starter culture for the next batch.