It's really simple, just copy and paste the PeerTube account's address into the search box in Mastodon.

The PeerTube account will show up in the search result section in Mastodon, then you just click on "Follow".

If you follow from Mastodon, when a new video is published it will show up in your main timeline just like Mastodon posts do. You can comment on the video by replying to it within Mastodon.