When you invoke the Time Tracker software you have downloaded to your computer, you'll see a list of all of your current hourly projects. Just click on the name of the project you want to work on and be sure to start the timer when you're ready. Don't forget to also stop the timer when you are not working on the project.
Even if a client says they will "allow" you to use manual time to add hours to your work time, DO NOT USE IT unless you can't invoke Time Tracker for some reason. A client never has any obligation whatsoever to actually pay you for any manual time you bill them for.