1. At the beginning of the text you want to hide, put the word spoiler in brackets, and the end, put the word /spoiler in brackets.
2. Use your mouse to highlight the text you want to hide, then click on the SPOILER TAG button, which is in the tool bar just above the text entry box.
As a tip, you can log in and hit the Quote button on any message that has a spoiler bar. You will see that the text entry box has the QUOTE tags and also the properly formatted spoiler tags, which will show you how it ought to look in your own messages.