There's nothing like the comissioning process to learn what the PFD and P&ID are all about. Walking the system from end to end and making note of the actual equipment and instruments installed in the field and corellating that with what's shown on the associated P&ID will give your understanding a good grounding in reality. You may well find there are some differences here and there....but that's another topic. (g)
For new processes, the development process often begins with developing a simple "functional block diagram" which spells out in narrative form the operations you want to perform on the media being processed, the input specifications and product specifications. Starting with a block diagram is nice in that it doesn't prematurely steer you towards one flow logic or another until after you've got your ducks in a row. But this is just my preference.
For processes I'm quite familiar with, such as oil and gas separation or compression processes, I'll dive right into the PFD or P&ID.
At the end of the day, the P&ID like many other engineering drawings, is a communication tool as well as a configuration control tool.
A lot of people don't really understand what a P&ID is. I've seen companies hand their shop a P&ID and say "build it". (g) The P&ID is NOT a design or fabrication drawing. It is an engineering drawing which is used as a primary point of reference for the design process. It's also a great operational and training tool.
Good luck!
Tom