How do I write an instrument definition file?

One way is to open any ins file and use it as a template for creating your own. Do not overwrite it but refer to it. 
 
From the user manual
 
Import Instrument Definitions dialog
To open the Import Instruments Definition dialog box, go to Edit > Preferences > MIDI - Instruments, click the
Define button to open the Define Instruments and Names dialog box, then click the Import button. The Import
Instruments Definitions dialog box contains the following fields:
Look in
Use this field to navigate to the folder that contains the instrument definitions you want to import,
File name
You can type the name of the file that you want to import, or click the file name in the window that’s just above the
File name field. Instrument Definition files have the extension .ins.
Files of type
The only choice in this field is Instrument Files ( *. ins).
See also:
“Importing instrument definitions” on page 1379
“Assigning instruments” on page 1376
“Creating instrument definitions” on page 1380

Hi
 
Here's a link to a video on Streamworks Audio's youtube page that shows how to do it.
 
HTH