Press the soldering iron tip against the circuit board's solder pad and the battery connector's lead for a few seconds to heat them both. Angle the tip so it has maximum contact with the pad and lead.

Feed the solder wire into the heated area until there's a concave pool ("mini volcano") of solder surrounding the lead.

Don't feed the solder directly onto the tip as it won't flow properly into the pad and lead, causing a weak connection.

Remove the solder wire first, then remove the soldering iron from the pad.

Repeat this process for the other battery connector lead.

If the second battery connector lead is no longer flush with the board, push it back through while heating the first lead before attempting to solder it in place. If needed, desolder the battery connector and restart this step.