Follow suit = do the same as everyone else has just done.

• If you keep your things tidy, the others may follow suit.
• If you sign the contract, others will soon follow suit.
• When one airline reduces its ticket prices, the rest usually follow suit.

Take the plunge = to make a decision or do something important or difficult that you have been thinking or doing for a long time.

• She wondered whether to enter for the race, but decided to take the plunge.
• If you have been thinking about buying shares, now could be the time to take the plunge.
• I am not ready to take the plunge yet.  Sam and Mary took the plunge.(2- to marry someone)

Go back to square one = go back to the beginning of a project, because previous attempts failed.

• I lost all my data when the computer crashed, so I am afraid  I’m going back to square one.
• The experiment didn’t work , so it’s back to square one, I am afraid.
• His idea didn’t work, so he had to go back to square one.