Singing at Sight Theory Things   William Wieland
  1. I choose a melody you have never seen.
  2. I ask, "What key is it in?" (Learn key signatures and how to tell major from minor.)
  3. I establish the key at the piano.
  4. I ask you to sing tonic.
  5. I ask you to sing the first pitch. (Learn to sing from tonic to any other pitch.)
  6. I ask you to sing the melody.

Melodia - Book I by Cole and Lewis (1904)   — an old, but FREE sight-singing book (Click "Read Online" in the left margin.)

Why sight sing?
An important attribute of the accomplished musician is the ability to “hear mentally”—that is, to know how a given piece of music sounds without recourse to an instrument. Sight singing, together with ear training and other studies in musicianship, helps develop that attribute. The goal of sight singing is the ability to sing at first sight, with correct rhythm and pitch, a piece of music previously unknown to the performer. Accomplishing that goal demonstrates that the music symbols on paper were comprehended mentally before being performed. In contrast, skill in reading music on an instrument often represents an ability to interpret music symbols as fingerings, with no way of demonstrating prior mental comprehension of the score. — Ottman & Rogers. Music for Sight Singing
Skill in being able to imagine and hear music inside one’s head from a printed score, which is the real goal of sight singing, is considered by many teachers, however, to be even more basic than the reverse process of writing down notes from played examples as in dictation. In fact, it would be difficult to imagine an activity that contributes more to the development of mature musicianship than sight singing. — Earl Henry. Sight Singing.
You should practice sight reading daily, just as you would practice your own instrument or voice. Steady, disciplined work will yield the best and longest-lasting results. Practice all examples only as fast as you can perform them with accuracy. — Benjamin, Horvit & Nelson. Music for Sight Singing.