This video offers a nice introduction on the comparative method, used to reconstruct languages without direct attestation, and then talks a bit about the reconstruction of Proto-Indo-European. It’s full of examples and rather accessible, even for people not well-versed in Historical Linguistics (or even Linguistics).
I love his videos. Besides PIE he has also a lot of good stuff on Proto-Germanic and Old English. Found the link for that one (by coincidence) on another comm and brought it here.