A Legato line is a smooth vocal line with no bumps, breaks or "h's". One of the most effective tips I found to get a smooth vocal line into a song is to practice the line on a gentle "edge" type of sound, usually on an "M" or an "NG." Recording oneself and listening to the playback while practicing in this fashion is usually quite revealing. You need to work on the line until the melody is as smooth as "oil on glass."
One of the most crucial aspects of singing Legato is to understand your breathing and breath management. While singing a legato line, you must learn a low breath into the lower back. I learnt a lot about mastering the breathing pattern and its effectiveness from Singorama. They have an entire chapter devoted to just breathing. Chapter 3 if I remember. I must say a perfect course to learn Singing.
I also learned a lot about legato line from watching Alan Lindquest and his student Virginia Botkin. Lindquest explained the proper position of the jaw as "hanging slightly down and back"; much as the jaw would hang down and back if one went to sleep with the head back. The jaw NEVER goes forward.
Kirsten Flagstad called the "ng" "the silver thread that is the soul of my voice". The "ng" must be produced with the root of the tongue wide. If the root of the tongue is "bunched", the singer will experience a "closed throat" and very tight singing. The correct production of the "ng" helps to lengthen the "vibrational time" of the vowels as well; thus a musical line can be accomplished much more easily.
More to come on Legato as I learn it!