Students do not see ANY of the exercises until they can sing them "by ear" without the teacher. That is, they have them aurally memorized. Choose solfège, scale degree numbers, letter names, etc. (it's your choice).
Students echo sing each measure individually. The teacher sings a measure and the students echo that measure.
Students sing the exercises in this way until they can sing each exercise without the teacher (that is, they have learned each one "by rote" or "by ear" and can sing it without prompting from the teacher).
Start back at exercise 1. Students see the exercises with just note heads (no stems) on a staff, but it is missing the clef.
Teacher points to the note as she/he sings echo sings each measure. The students should catch on that this is the same as the first exercise (and so on). As the students sing, the teacher points to the pitches.
Start back at exercise 1. Students see the pitches on the staff that includes the clef (treble clef, bass clef, or both).
Include barlines and stems. Have them also read the rhythm (they should have doing rhythm separately to this point).
Go through all exercises this way and then transfer to other key signatures and repeat (this should go fairly quickly).
Transfer to "real" music (their concert literature or other pieces for sight-singing practice).