Wake up on your fast day and consume a maximum of 500 calories (for women) or 600 calories (for men) until you go to bed. Wake up the next day and eat as normal! Do this two non-consecutive days each week.
It doesn't matter how you spend your calories or when you eat them, though it is recommended not to graze/snack and to have them as 1-3 meals. Personally I find I get ravenous once I've started to eat on a fast day, so I'm best saving my calories for a nice big, filling dinner. Others find a breakfast suits them better, or a small meal 3 times a day.
Some of us like to begin our fast after dinner the day before a fast day, though that's not essential, it just increases the amount of time spent without food.
What is the 5:2 fast diet?