How do I define or edit fields?
You'll add and edit fields in the SeedCodeData file. Find that file in FileMaker's window menu when Complete is open, and then select File / Manage / Database to create and edit fields.
Linking tables to each other is usually done in the graph in SeedCodeComplete (the main file). The exception would be relationships that are needed for lookups and calcs: such relationships need to be defined in SeedCodeData, though they may also be expressed in the main SeedCodeComplete file.