Step 1:
"Vibratto" is an Italian word and it means vibration. Using Vibratto when you sing means "the vibration of the pitch/note". Just like a guitar string vibrates to create its own unique tone, so does the violin player, cellist etc. If you ever watch a violinist or cellist you may notice the back and forth motion that they make with their wrist that is actually on the strings. This motion is "VIBRATTO". The vibratto technique adds a very warm, pleasant and resonant tone to most instruments. This includes your voice (also an instrument).
Step 2:
You can recognize vibratto when you hear it. It sounds like a mild and controlled trembling in the voice. To practice using your vibratto, you want to start the trembling motion slow. This will help you to get used to where your vibratto is coming from. Once you are familiar with where the vibratto motion is coming from on YOU, then you can control it better and then work on speeding up the vibratto.
Step 3:
Choose any two notes that are next to each other in pitch. (Ex. C & D or F & G.). Using a vowel sound like AAH or ooh, go back and forth between the two notes about 8 times and then try to maintain the trembling that you have started on the last note. Repeat this and then choose different notes (remember they have to be right next to each other). When you feel that you are improving, you can start speeding up the vocal-exercise. To develop your vibratto, you have to be consistent with this vocal-exercise. Once a day for ten minutes is good. To hear a sample of vibratto, go to the link below and listen to samples of "Ready To Sing VIBRATTO"