There are many different methods for installing InspIRCd. You will need to pick the one which fits your needs best.
To build from source:
Source Installation — a guide to the methods through which you can obtain a copy of the InspIRCd source code and build it on your UNIX-like system.
Extra Modules — a guide to adding extra and/or non-standard modules via the module manager.
The following operating systems have prebuilt packages available: