• The Shabbos lights should be lit at the usual time (in America, for instance, 18 minutes before sunset), preceded by the Chanukah lights.
• In Jerusalem, the custom throughout the year is to kindle the Shabbos lights 40 minutes before sunset, but on erev Shabbos Chanukah they are lit 25 minutes before sunset after the lighting of the menorah. (Some Jerusalem families kindle the Shabbos lights at the usual time, preceded by the Chanukah lights.)

• If it is still before sunset and he has not yet accepted Shabbos, he should relight it without a blessing.
• If he has accepted Shabbos, he should ask another person who has not accepted Shabbos to light it for him.
• If it is already sunset, nothing can be done. The obligation has been fulfilled even if all the lights go out.

Excerpted from"Guidelines" - more than 200 commonly asked questions about Chanukah (Targum/Feldheim).