Double-click on a video or audio clip in the project panel to open it in the source monitor.
Place the current-time indicator in the sequence you are viewing in the source monitor exactly where you want your clip to start.
Select the track(s) to the left of the timeline where you would like your clip to overlay. Clicking on the track(s) titles such as Video 1, Video 2 or Audio 1 until highlighted gray will select the track.
Drag the source clip track indicators to the tracks where you plan to overlay the clip.
Click on the overlay button or the overwrite button in Premiere CS5 and CS5.5. Both are just below the source monitor and indicated by a symbol of a downward arrow over a gray line. The clip should now be overlaid to the track(s) you selected.