You should floss in the morning and at night right after you brush your teeth. Dentists also recommend you floss after every time you eat as well but I believe that as long as you do it at night to get rid of food particles stuck between your teeth that you gather throughout the day before you go to bed, then you should be okay.
Flossing should be done at least twice a day. It helps with keeping the gums healthy and clean. The best times to floss would be after eating. Be sure to have the floss go all the way to the gum line. It is important to floss correctly, if there are any questions, ask your dental hygienists.