Listen to the melody by yourself or play it on a keyboard to become familiar with it. It will be easier to teach the melody if you know it well.
Hum "ooh" or play the melody for the children multiple times.
Divide the melody into short phrases or lines, then hum "ooh" or play the first phrase a few times for the children.
Have the children hum or "ooh" for the first phrase until they are familiar with it. Repeat with remaining phrases.
Hum or "ooh" the whole melody four or five times with the children.
Say the words, phrase by phrase, and have the children repeat them if the melody has lyrics. Then say all the phrases together.
Put the melody and the words together, phrase by phrase, and sing the whole song a few times until the children feel comfortable singing it by themselves.