Click Set button to set the reference movie to the movie you want to add a subtitle track.
Click the upper-right Add button to add a new text track.
Start playing the reference movie.
When the reference movie reaches the point you want to add a subtitle, click the lower-right Add button. A new sample with the current movie time is added.
Repeat step 5 until all subtitle are marked.
For each sample, type the subtitle.
The reference movie is set to the sample time when a sample is selected. You can fine tune the sample time by stepping forward or backward, then click the Update button.
Make sure the text track is selected.
Select menu item Track Info to bring up the Track Info window.
Select Dimensions of the popup menu on Track Info window.
Position the text track by adjusting the coordinates.
Switch to Styles.
Change Font, Size, Style, etc.
Switch to Drawing.
Change Alignment to Center.
Enable Keyed Text.
(Optional) Enable Drop Shadow, and set Drop Shadow Offset to 1, 1.
Save the project.
Click Preview to preview the resulting movie.
Select menu item Export to export the resulting movie to QuickTime movie.
Click to view sample movie
You can download the sample project here. (Please note that you have to re-set the reference movie to a local copy.)