Every thread has a link at the top which says Watch Thread. To watch a thread, click on that and a box will come up and ask you to confirm you want to watch the thread. Unwatching works the same way.
If you post a comment in a thread, there is no need to do the above--it automatically becomes a watched thread.
Then, when you want to see all of your watched threads, click the Watched Threads tab at the top of the website. I've made a bookmark on my device for watched threads which would be the same for anyone on the forum. This is the link:
https://www.websleuths.com/forums/watched/threads