Unfortunately, that's like asking how do I design a house. You should certainly cruise the technical information side of the various manufacturers' web sites, but those are more about the tools, not the design.
The first question is always "What does the client think of as 'security'?" To some it's a simple burglar alarm, to another it requires motion detectors above the drop-ceiling and time-of-day controls on the access card readers.
Design elements will include (not limited to..)-
number and type of doors (door construction, egress, entry, etc)
overall building construction
will the sytem call a central station? phone or phone & radio backup?
must it monitor a fire alarm panel?
number and type of access card readers
etc