We can create a Project when an Estimate is created using 'Custom Function' and 'Workflow Rules' in Zoho Books

1.1 Go to 'Settings' --> Tap on 'Automation' 

1.2 Select 'Custom Functions'

1.3 Tap on 'New Custom Function'

1.4 Fill up mandatory fields

1.5 Paste the code from 'Code Snippet' and tap on 'Save'

2.1 Go to 'Settings' --> Tap on 'Automation'

2.2 Tap on 'New Workflow Rule'

2.3 Fill up mandatory fields and tap on 'Next'

2.4 Select 'Created' from 'Choose when to Trigger' and tap on 'Next'

2.5 Criteria is optional.  If you need to set any criteria, you can choose it. Then tap on 'Next'

2.6 Tap on 'Type' from 'Actions'

2.7 Choose 'Custom Functions' from 'Immediate Actions'

2.8 Choose the 'Custom Function' which you created in step 1 and tap on 'Save'