Check out the YouTube channel called Cassetteboy to see how this works and for further ideas.
https://www.youtube.com/user/cassetteboy
However, for this lesson just downlead the files below, watch the video and make your own Donald Trump speech. There is no right length of speech but it needs to be more than just a few seconds, if you really want to challenge yourself then add some subtitles or credits at the end.
How to make a video mashup using wevideo