Regardless of whether the customer is internal or external, this how you collect the requirements:
- You meet with the customer/client and you ask him about this business and the project that he wants to get done. You try to understand what his business (department in your case) does, so that you can better understand what he wants. You do your best to understand the relationship between the project and his business. If you can't understand the relationship then you should ask him again.
- You research (online) similar projects for similar businesses and you make a list of what his requirements should be.
- You then ask him what he wants to get done. You document all his requirements. You then suggest the requirements (that he didn't include) from the list that you created in the previous step. He might approve some, he might reject some, and he might change/drop some of his previous requirements.
- Once the above step is over, you go back to your office and you review the requirements again, and then do one of the following:
- Suggest that he adds more requirements because they're missing
- Suggest that he removes some requirements because they won't benefit his business
- Suggest that he changes some requirements to better accommodate his business
- Once you have a better version of the requirements, you then start looking at other similar projects (undertaken by your company), to see if other clients/customers asked for different requirements, and you go (again) through the process of suggesting to add/remove/change requirements when you see fit.
- You then talk to your project team, and get their feedback about the requirements to see if they are all feasible or not. If some of the requirements are no feasible, then you have to get back to your client and suggest an alternative.
- You then draft a final version of the main requirements, you get it approved by the client, and you create your project charter based on these final (and approved) requirements.
It's very important to keep the client very close during this stage: as you can see from the above, we could have gotten the customer to re-assess his requirements only once, but we did it 4 times, just to keep the client close and because it's much better to follow an iterative approach when collecting requirements.