Open your preferred digital audio workstation -- for example Logic, Pro Tools or Cubase.
Open the "File" menu, click "Open" and browse for the session to be edited. Click on the session name, for example "Party Tune in progress 1."
Hit "Play" and click the "S" icon on each channel strip in turn. The "S" stands for "solo." When you solo a track, you mute all other audio. This helps you determine what audio is on what channel. When you find the track with the sub bass on it, double-click the track name and type "Sub Bass." Click "M" to mute this track.
Press "Pause."
Right-click the audio and select "Copy."
Click "File" and select "New Audio." Right-click in the new audio track and select "Paste." Rename the track "Sub Bass Drop."
Click on the "Sub Bass Drop" channel to highlight it. This assigns subsequent edits to this track only, rather than the session as a whole.
Click "View" and select "Zoom." The audio appears as a sound wave graphic, with peaks representing high volume. By zooming in, you expand these peaks, making it easier to cross-reference the energy patterns of the sound wave with the sounds as you hear them.
Solo the track "Sub Bass Drop."
Press "Play" and let the track roll to the point where you wish to insert the bass drop -- for example, at the end of the breakdown just before the last chorus.
Select the "Trim" tool. The exact process for doing so varies according to which program you are using, but it is typically in the "Tools" menu.
Click to the immediate left of the sound wave peak representing the first bass note to be removed. This inserts a trim point. Click to the immediate right of the sound peak wave representing the final bass note to be removed. This separates the selected audio from the track. Right-click on it and select "Delete." This removes the selected audio, but leaves the remaining audio intact. The resulting gap in the audio creates your sub bass drop.