Custom Validation

I want to create a custom validation on a form. I’ve read the developer’s documentation on custom validation, but don’t understand A- where to input/edit the code in the editor or B- how to apply the change to only one of the forms that I’m using the plug in for.

Specifically, I want to create validation that on submission of the form, it counts the number of checkboxes checked and either submits the form or displays an error message based on that number. I’m using Contact Form 7 for multiple forms (including another on the same page).