How to concentrate to suppress hindrances? I heard one needs concentration to abandon the hindrances -- however to have strong concentration, you need to have [already] abandoned hindrances, so that nothing can stop your focus. Which one is it? How do I abandon the hindrances: one hindrance at a time or all together?
Kusala means  minds that arise to abandon chance to arise of akusala-minds. While kusala arising, akusala-minds can't not arise ( just in the same person).

3 sikkha = adhisila-kusala-training, adhicitta-kusala-training (concentration meditation), and adhipanna-kusala-training.

So, the practitioner practices adhisila-kusala to abandon a chance of  akusala arising that form action/speech, practices adhicitta for just  some other akusala arising--such as beauty-attachment (akusala) by asubha-adhicittasikkha, etc, and practices adhipanna for whole akusala arising--11 khandha such as the past khandha, etc.

While whatever sikkha arising, no hindrances, no akusala arising.