Forget the model!
aka: Start with the Process Area names.
I had a tough time coming up with an appropriate title for this entry because of the simple point I want to make...
When/if you're just getting started with your "CMMI journey", don't obsess over the practices or their goals, just look at the names of the process areas and ask yourselves the following question: WHAT do we do in this area?
NOT "Do" we do this area, yes/no? But, "what" do we do?
EVERYWHERE & ANYWHERE it might show up throughout the development processes of your organization.
I would not expect any one PA to show up in any one place, in fact, I wouldn't limit my expectations for where to expect PA "stuff" to show up, so, to widen the net for "what" you do, I'd toss the question out to as much of the development participants as I could from PMs and TMs to developers and testers and product mgrs. Where possible and related to a PA, I'd even seek input from customers.
You need to know what you're doing before you should try to improve it. Too often I find folks trying to dive into the practices in a PA before actually knowing what they actually do.
You can read the purpose statement of the PA to get a sense of it, because your organization might not call what you do by the same name, but from that point forward, until you've described your actual way of performing the PA -- on name alone -- don't get all caught up in the PA's goals and practices... BECAUSE those are meant to IMPROVE your processes, NOT DEFINE them.
If you aren't doing anything in a PA, that's another story. But, my suspicion and go-in expectation is that you've gotta be doing something or you wouldn't be in business.
More on that another time.