Some Considerations...
Some suggestions to help you on your way:
1) Try to find a written 'Operating Manual' or general project documents related to the machine. This will often have a general machine description or 'Theory of Operation'. If not available onsite, a friendly call to the machine's fabricator will often be very useful. If not abvailable, Google the machine name or description and see what happens; you might get lucky. Also, sit down with a knowledgeable operator or maintenance person to interview for an operating description... be thankful and humble.
2) You have the schematics, so I assume you'll have a list and description of the I/O (inputs/outputs connected to the plc); this is very important info. Understand what each sensor and actuator actually do and where it's situated on the machine.
3) Get manuals of the specific PLC you're using, particularly the instruction set, data types, and hardware manuals. Hopefully, there're online manuals.
Good luck.