A BlueCollar Contract / Sales Order can be created through any native means. This includes creation from an Opportunity or from a Quote/Estimate. Additionally, a common flow is to create the Contract from the Project record that was created previously.
Regardless of the method used, what's important is that the required fields are exposed and filled out. See Sales Order / Contract Form Configuration for more information.
When creating a Contract from the Project, select the button on the Project record that says 'Create Contract'. This will open a Sales Order form and pre-fill information from the Project record such as the Project Name as well as the customer.
Once the Contract is saved it will be linked in the Contract / SOV field on the Project record:

Alternatively, if a Sales Order already exists and is not yet linked to a Project the Sales Order can be marked as a Contract and Associated to the Project from the Sales Order form, which will subsequently link the Sales Order to the Project in the same way.