Keybind it in Options and ---HOLD--- the key to use it. Releasing the key will make it disappear. The GUI will appear over or below your chat GUI (was designed for 5 lines of chat, minimum chat size, and no chat timeout).
The Timer Start/Stop and the adjacent Chat Start/Stop start an upwards-counting timer. The timer will display tenths of milliseconds in bottom print and milliseconds with decimal in the GUI. The Chat options will announce the timer's starting and stopping. The Countdown Start will start a downwards count from the value in the text box. Chat Start announces it. The Talk on End function will announce the countdown's stopping. Countdowns are only displayed in the GUI.
There are also shorthand commands:
/stopwatch (or /s) start (or s) will Chat Start a timer. /stopwatch (or /s) stop (or end or e) will Chat Stop a timer. Append Covert or C to an argument to non-chat start/stop. (/stopwatch covert start (or cs))