While I personally have not done it, I've heard it's best to soak about 2oz. of oak in the wine, and to change the wine every 2-3 days. This will extract some of the oak tannin, mellowing it out, and allowing it to soak up the wine.
If you're like me, though, that's a lot of wine that you could be drinking. I'd say boil the oak for 5-10 minutes, strain off the water, and add the oak with the wine into the secondary.