What is the first step in balancing a redox reaction by the half-reaction method?

First step is to balance the amount of atoms on each side

So for example if my equation that I have to balance is : Br2 -> Br-

First thing I would have to do would be :
Br2 -> 2Br-

Then I would have to balance the charges on both sides. So because there is a -2 charge on the right side I will have to add a -2 charge onto the right side as well. I would do this by :
Br2 + 2e- -> 2Br-

And now you can see the amount of atoms and the charges are balanced... So your overall reaction is now balanced too.

It would be (at) + cu (s)