As of the time of writing, IDA does not have a built-in plugin manager, so third-party plugins have to be installed manually.
Installing into IDA directory
The standard location for IDA plugins is the plugins directory in IDA’s installation (for example, C:\Program Files\IDA Pro 8.0\plugins on Windows). So this is the most common way of installing them — just copy the plugin file(s) there and they’ll be loaded on next start of IDA. However, this only makes them available for this specific IDA install. If you install a new version of IDA (which by default uses a version-specific directory name), you’ll need to re-copy plugins to the new location.
Installing into user directory
In addition to IDA’s own directory, IDA also checks for plugins in the user directory. So you can put them in:
%APPDATA%\Hex-Rays\IDA Pro\pluginson Windows$HOME/.idapro/pluginson Linux/Mac
You can find out the exact path for your system by executing idaapi.get_ida_subdirs("plugins") in IDA.
Such plugins will be loaded by any IDA, so there may be issues if they use functionality which is not available or changed between versions, but the advantage is that there’s no need to reinstall them when upgrading IDA (or using multiple versions).
See also:
Igor’s tip of the week #33: IDA’s user directory (IDAUSR)
IDA Help: Environment variables
