User documentation is in the section called “Sing interval”.
Here is a minimal lesson file:
header {
module = singinterval
lesson_id = "a400df62-e007-4a1b-9057-cd05397e88a2"
version = "3.1.4"
title = "Thirds"
ask_for_intervals_0 = [3, 4]
test = "3x"
test_requirement = "90%"
}