Before teaching a new song, it is always required to pre-teach the targeted vocabulary. For this song, that would be
sun, rain, rainbow, and flowers. Prepare the flashcards and introduce them to your students. You can use some of these
FLASHCARDS GAMES IDEAS.
Depending on the English level your students are at, you can also add warmth, sound, sight, or whatever you like. It is always a great idea to use TPR as well.
After introducing the targeted vocabulary, ask students what will appear in the sky when the sun is shining and it is raining at the same time. They will probably know the answer: the rainbow.
After the conversation, sing the song using TPR. The second time, ask the students to do the TPR as well.
To boost learning, you can make for example several cards of each word you have introduced and give them to your students. Ask them to stand up each time when they hear their word. Or, you can make prompts sun, rain, rainbow, flowers, like we did, and "dramatize" the song as if it were a story. Well, it kind of is.
For the follow-up activity, ask your children to draw "How Is A Rainbow formed?".