How do I start a data flow diagram for a business process?

For example, if I'm in a call center and need to reset your password of a client.

There are tons of websites and other programs available for this exact process. My best recommendation is a process called mind mapping. I use a site called bubbl.us and I like it! The best part is that you and your partners or co-workers can edit and update the map of the mind. Check out my box of origin of a Bubble such a mental map. Excellent luck, Alex