Using soured beer in stout

I just read the article on the history of stout in BSHN news.  Great article and adding the soured beer to a stout recipe really intrigued me.  In the article, after you sour the beer over the course of several days, you sterilize through boiling (what is a good range of soured beer amount for a 5 gallon recipe and how long to boil) then add it to the fermenter.  Should you add this to the primary fermenter before racking it to the secondary, add to secondary or does it really matter.  Should I add it to the primary and let it sit in the primary for a few days before racking to the secondary even though most fermentation would be accomplished after several days of that it takes to sour the beer.

Thanks!

I've heard a variety of different methods for this. I've taken 24 oz of Guinness and left it out in a bowl for three to four days before brewing. This amount was for 5.5 gallons.

On brew day, I'd bring the "soured" beer to a boil and then add at flame out. I suppose you can just bring it to a pasteurizing temperature somewhere around 170-180? I'd be curious to bypass the flameout addition and add it in secondary to compare.