Several ways to do it really...
One way would be to select Options/Quick-punch...then place the cursor on the time-line exactly where you want the punch to begin. Put the track in record. You might want a few bars of pre-roll (on the transport) so you can get locked in before the punch. Hit record, play over the section you are punching in on. Hit stop. You're done.
It may be easier to select a piece of audio also. Highlight the whole area you want to record over (maybe it's 4 bars long or whatever). Again, use the appropriate amount of pre-roll, hit record, play the part and you're done.
I believe you can also use the Loop Record function and get each pass to create a new playlist on your track and then you can record several passes at once and then go back and "promote" the best one.
