How did you find fasting the last time round? It is a difficult thing to do if you are not well practiced. Pick the best day this week again, when you are not too active, and fast for the whole day. Ignore the cries of your stomach, they are much like the cried of a spoiled child. Starvation does not kick in until around 2weeks of no food so a day is little! Do drink water.
This discipline will come around once a month in this challenge. If you feel you can do it more often then great. If, for health reasons you feel you cannot then no worries. Perhaps replace this discipline with cutting out something else in your life such as a TV programme.