To develop the main melody/tune/vocal line for your song.
To develop the main hook or riff for your song.
Developing the vocal line
Improvise lines of a tune, or to rap verbal phrases in a rhythm which fits your chord progression.
Try putting words or phrases to it.
Repeat this sequence, try new ideas which work with the chord progression.
Developing your Chorus
Your chorus is your hook you need to make it so you can sing along to it and make it interesting.
Repeat one or two lines in the chorus, or repeat a line with a very slight change (perhaps in the harmony). Include a line from the verse in the chorus.
Check what you have is a chorus. Could it be a different section?