Introduction about Powershell execution policies: get-help set-executionpolicy
First a signing certificate matching a specific template. It could be a self-signed certificate but it does not really make sense.
Relying on PKI infrastructure, there is already a “Code Signing” template. For instance, have a look to:
In the same link, there are instructions to sign the PowerShell script with the generated certificate.
But PowerGUI provides some add-on feature to make the life easier: