Excellent means really great work, work that shows effort, imagination, thought, and special care.
To get an excellent follow all the instructions on the task and the process and be sure that:
Very good means all parts of the unit have been attempted, but it shows some deficiencies such as that:
Good means that not all parts of the unit have been developed or that they have been developed poorly:
Fair means that only parts of the unit have been attempted and that they have great deficiencies: