Summarize this article:

Go to https://www.facebook.com/ in your computer's web browser. This will open your Facebook News Feed if you're logged in. If you aren't logged into Facebook, enter your email address and password before proceeding. Scroll down until you find the post or comment for which you want to post an emoji reaction, or go to the profile of the person who posted the content you want to open. It's below the post or the comment. After a second, you should see a pop-up menu with different emoji appear. Click one of the emoji in the pop-up menu to apply it to the post or comment.  You can only react with one emoji at a time. If you just want to add the "Like" thumbs-up icon, you can click Like.

Summary:
Open Facebook. Go to the post or comment to which you want to react. Place your mouse cursor over the Like button. Select an emoji.