Using the timer.

To time a task, select the task in your list, and click on the Timer button in the Tasks tab or the Time tab on the ribbon:

This will bring up the timer:

The timer will start timing the selected task.  You can start and stop the timer by pressing the Stop/Start button.

You can add a note to your time entry by clicking on the Note button.

You can start timing a different task by clicking on the "Select a task..." button.  Tasklist will stop timing the current task, and start timing the task you select. There is no need to stop the current timer or to save the time.  The description and number of the task being timed appears next to the Select a task button.

You can change the time by stopping the timer, clicking on the time element you want to change (hours, minutes or seconds), and typing or using the up-down arrows to change the value.

You can make a new time entry without using the timer.  The topic Editing your time explains how to do this.