Agree the delivered scope: source repository, editable assets, build instructions, test notes, exported builds, and rights records as appropriate. Keep secrets and signing credentials separate from ordinary source files and use an approved exchange method for sensitive access.
Test the instructions from a clean starting point
Where practical, have someone other than the original developer follow the setup and build steps. This can reveal missing files, local paths, undocumented settings, and assumptions that were invisible on the development machine.
Document the exact versions and known limitations. Explain how content is organized, where configuration belongs, and how to run the relevant checks. Godot's export documentation, for example, distinguishes project packaging from platform-specific setup; the receiving team needs the workflow for the actual project.
Keep the final source revision linked to each delivered build. If a later change is made, the team should be able to identify what it includes. A reliable handover does not promise that future upgrades will require no work. It gives the next maintainer a concrete starting point, an honest account of the current state, and a way to reproduce the version the client accepted.
Source: Godot export workflow reference.