Before the module installer can be used, the PSSModuleInstaller module must be imported into your PowerShell session like so:
Import-Module PSModuleInstaller

To inspect the functions this module exports type:

(get-module PSModuleInstaller).ExportedFunctions

Once the PSModuleInstaller module is imported, you can learn how to use the commands with the PowerShell Get-Help cmdlets. For example:

Get-Help -full New-PSModuleInstaller