Bring up the app. Modify the Starting Hobbs time (if you want to, I don't think it's critical; FYI, the app will fill in the Starting Hobbs field with the value of the Ending Hobbs field from your last flight). Click on the Engine Start field (that, I believe, is critical) to have it fill that field with the current date-time -- or set whatever date-time you want. Then MyFlightBook will stay running in the background. (You don't need to keep it in the foreground -- for instance, you can switch over to iFly). It will enter your Flight Start time by itself based on what you have noted as your takeoff speed (click on the human figure icon along the top line, then scroll down to set that one time), and will put your origin airport in the Route field. It will note any other airports you go to in the Route field, and will enter your Flight End time automatically. It's up to you to enter Engine Stop time and Ending Hobbs time, assuming you want to. It will also count your full-stop landings (and whether they were day or night). If you've specified such, it will also automatically fill in your Total Time (and whether day or night). Works quite nicely. Occasionally it mis-counts the numbers of landings.
You probably also know that you can enter/search/modify data on the app or on their website. I do the actual Engine Start, Engine Stop, and Ending Hobbs on the app, while in the plane. When I get home on the computer, I make sure everything adds up right, and add comments.