Okay, insert a new video track so you have at least two, put the main video on the bottom track and on the second track place the one you want as the mini clip in the corner, make sure youve edited these to your liking etc first though. Once you have the second clip on the second track, click the track motion icon, click the top right handle on the editor and drag it to make it smaller, once you have the size you like, click within the middle of the square and drag to position it where you want it.
The track motion button is this one circled:
