In the text editor, there's a button (3 to the left of the smilie button) for "Insert...". This button gives you an array of things to insert ... one of which is a 'spoiler'. Clicking on spoiler, will open an overlay where you can type the title of your spoiler. This is the visible text that will appear on the button. After doing this, simply type your spoiler text within the spoiler tags. Once complete, a reader will need to click on the spoiler button in your post to view the text that you've hidden.
Another way to go about this is to type your spoiler text in your message, highlight that text, and then click on the spoiler button.
It's also possible to nest spoiler text within each other.