To add your reaction to any message:
Hover over the message you want to react to. On the far right-hand side, select the smiley face with a plus sign.
A window of emoji reactions will pop up.
Select the reaction(s) you want and they will show up below the message.
Add multiple emojis or remove reactions by clicking on it again.
To add mobile reactions to any message:
Hold down the message you want to react to (press your finger down gently on the message)
A list will appear, which will have "Add Reaction" at the top. Selecting that option will allow a reaction window to appear, where you can select an emoji from.
Here is what it will look like: