Play it by ear → 见机行事、随机应变

Play it by ear (phrase) - proceed instinctively according to results and circumstances rather than according to rules or a plan. 

同义词: improvise, wing it

They are just playing it by ear. → 他们只是见机行事。 

We're not making a plan, we're just going to play it by ear. → 我们没有制定计划，我们只是随机应变。