macOS Problems building Growl.app from source

[tinybruk]

Plz bear with me, as a newbie to XCode builds.

OK soI have the Growl.app source code project for the latest build.

Trying to build the project I get

Code Sign error: The identity '3rd Party Mac Developer Application' doesn't match any valid, non-expired certificate/private key pair in the default keychain

in XCode

The Growl webpage states

A signing error. You will need to generate a self signed certificate, and can do so at Keychain Access.app -> application's name menu -> certificate assistant -> create a certificate. Be sure to use the same prefix as the certificate that you are receiving errors about.

I'm confused here as cant seem to follow the above instruction correctly.

Any help appreciated in step by step if possible