Could you please point me to the documentation/instructions for including an external file (one I wrote) in an open sketch?
What kind of file?
Typically, one creates extra .ino files to contain functions that the sketch calls. All .ino files in the sketch directory are opened in separate tabs when the sketch is opened, and are all combined into one .cpp file for compilation.
You can also create header files (.h) and #include them in a sketch. Then, you create a correspondingly named .cpp file in which the functions defined in the header file (if there are any) are implemented.