You may find these PatrickJMT videos helpful for learning integration by substitution:

“Integration using U-Substitution”:

This is from a YouTube math channel I like, called PatrickJMT. You can find his playlist for integral calculus here. We will be covering most of those topics this semester, so you may want to revisit that playlist over the course of the semester.

If you prefer Khan Academy, see these videos in their calculus course. In particular, here is Khan Academy’s playlist on substitution, which starts with the following video: