Sour Mash questions...

[pabst311]

so i'm currently running my first fermentation right now. the recipe is as follows:
6lbs flaked corn
2lbs flaked rye
2lbs 6-row pale malt
1lb raw sugar
Added in some alpha amalyse at 155*F
pitched american ale yeast starin from wyeast and 6 beano tabs at 75*F.
been fermenting fro about 1.5 weeks on the grain. i kept the fermentation vessel sealed and had a airlock in place to keep any funky stuff from getting in and infecting the fermenting mash.

i'm planning on doing sour mash in the future after this initial fermentation. this is where my questions and confusion come into play:

i'm going to run my product through off the grain. once i'm done running i'm going to keep about 1.5 gallons of the backset and add this into a new fresh batch of mash which i will start the same day. The addition of the backset is what will create a sour mash (Correct?). is my understanding of what i've read correct or am i totally off my mark? should i re-use some of the grain from my first generation as well? i'm under the impresion that in order to create a sour mash u pretty much want to have some of the bakset/grain "spoil". is there anything else i should keep in condiseration when doin sour mashs?