Context: Bad Lip Reading (abbreviated as BLR) is a YouTube channel, run by an anonymous individual in the music production business, that spoofs clips from films, TV shows, songs, sports, and political news stories by overdubbing humorous vocal work that matches the lip movements of the targets. Rolling Stone described the channel as "the breakout hit" of the 2012 United States presidential cycle. As of January 2018, the Bad Lip Reading YouTube channel had amassed over 6.5 million subscribers and over 942 million views across its videos.

Question: how do they make bad lip reading videos?
Steam of consciousness below:
To answer this question, we should know that: Bad Lip Reading (abbreviated as BLR) is a YouTube channel, run by an anonymous individual in the music production business, that spoofs clips from films, TV shows, songs, sports, and political news stories by overdubbing humorous vocal work that matches the lip movements of the targets.
So, the answer is overdubbing humorous vocal work that matches the lip movements of the targets.