Go to a forum, perhaps by using the dropdown menu labeled Forum Jump at the bottom of a thread (such as this one). (Another way is to click on the Forums button at the top of the FlyerTalk home page.) Once at the forum of interest, find a dropdown menu labeled Forum Tools (in the same blue-background line as Threads in Forum: [forum name]. From that Forum Tools dropdown menu, choose Subscribe to This Forum.

This assumes you are not using the mobile skin and something else that eludes me at the moment. In other words, I assume you are using FlyerTalk the way that I do. Good luck, and come back and ask me to elaborate on what I've left out. If you're like me, you'll want to open the forum of interest in a new tab, so you can refer to these instructions.