The above is a picture of buttons above the reply writing box
Spoiler is a feature that hides text, so that it requires the user to actually click the spoiler to view it's content. You use this so it doesn't spoil anything, unless the user wants to know.
All you have to do for this, is highlight the text, click the spoiler button and enter a title. If you just want the button to show without the custom title, leave it blank and press 'Continue'.