Creating the Template
You can create the template as described in topic Creating the Template.
Viewing the Template
You can view the template by clicking the View button following Body Template.
Editing the Template
You can edit the template as described in topic Editing the Template.
A JSON template normally contains a Start expression that controls which data rows are present in the resulting JSON file.
<<Start: Select(MyTable[MyKey], TRUE)>>
"Computed Total": "<<[Qty]*[Price]>>"