To add your reaction to any message:
Hover over the message you want to react to. Four icons will appear on the right side of the message. 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.
If there are already emoji reactions to the message, the same smiley face with a plus sign will appear next to the already existing emoji responses. Click this for a quick way to add your own emoji reactions.
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 popup will appear, select "Add Reaction". Selecting that option will allow a reaction window to appear, where you can select an emoji from.