Use EFT and set up a day in the near future to start the fast.
Every day prior to that day you use the EFT as often as you can to reinforce your intention to fast starting on that day. You can include the length of the fast, and what kind of a fast it will be. Doing this you have already decided on what you are going to do. You've removed any doubt from the start a couple of days in advance.
I find it easier to start in the morning after being without food for the previous night.
Every time you feel hunger coming on, you do EFT and reinforce your intention to fast.