This is simple to do. Open the facebook app in your smartphone.
Scroll down to see a variety of posts, images, videos and many more.
You can pass your reaction like "Like", "Love","funny", "sad" etc by holding the like button and 6 different reaction faces can be seen.
You can choose any one of them and pass your react to the post.
For example, the above image shows the 6 reactions. Now choose according to your choice.