Modbus Master (arduino uno), and simple modbus slave pressure sensor

Hello All,

I need some help and I would appreciate any assistance.

I am trying to program arduino uno to read pressure value from holding register of a sensor that communicates in modbus RTU.
I have purchased max485 shield, so I do have the interface between ttl to rs485.
I read the the modbus master library, I could not quite understand how it works,

Where should I start and where can I read more?
I am familiar with modbus and data packets.

Thank you,