Drain,
I am assuming that your experience level is quite limited in this area, therefore I can provide some info, but you would also be well advised to discuss this with some more senior individuals in your office regarding how they like to develop their P&ID schematics. I say this, because there is some personal preference in this area.
Firstly, prepare a logical flow diagram of your entire process. Break it down into the major components and determine how you want to identify each process. For example all piping and valving concerning raw water shall be "A" or "R" or whatever you choose and then define the piping sequence label (10, 100, 1000) etc. Your instrumentation and valving should fit within these general label for ease of reference and logical sequencing in the process.
Secondly, arrange the drawing in a logical format, being careful not to cluster or crowd too much. With today's technology, you can always cut, but it takes more time to split up a cluttered process sheet. Leave plenty of room between processes or components to add piping, valves, flow controls, instrumentation, etc., and their requisite labelling.
Thirdly, seek out assistance or older P&ID's generated by your office to get a feel as to what it should look like and you firm's preferences. This will include use of colour, layout philosophy, process identification and legend table.
Forthly, there are a number of good software programs that provides valuable assistance and allows the entire process to be much easier. Cost will be a consideration.
KRS Services