Then once you log on to the game, you’ll want to make sure that the addons are enabled via the menu option on the character screen.
Like with addons in the current game, you just need to drop any of these addons into the addons folder - find it under /Interface/AddOns/ in the Classic folder, which is in your World of Warcraft folder by default. Keep in mind that the client runs on the Legion API, so there are a few semi-current addons that work without any tweaking by their creators. The site has a list of addons - including a search function - and has color-coded each one to signify whether or not it’s working yet. The easiest one to parse through is Will It Classic. There’s a whole list of websites that are trying to compile the working WoW Classic addons. Thankfully, there are already a bunch of addons that work with the game, and addon-makers are creating new ones every day.
If you’re planning on playing WoW Classic, you might feel a little naked without your addons.