pretty simple. mark the in and out of the video clip in the timeline (mark clip) and go to the in. make sure the clip is selected and "match frame" which should load the source clip in the source monitor. make sure the patch panel is set up correctly so the source audio is on and the target audio track of the sequence is matched to it and then overwrite...
I'm using a keyboard preset based on the fcp7 keyboard so I haven't included the keyboard shortcuts...