The spoiler-tag is the eye-icon next to the smilie face. When clicking on it, a box much like the quote box will open. Type the spoiler into the box then hit "submit reply". The spoiler will automatically be hidden from view. If there's a non-spoilery part to your post that preceeds the spoiler, please type that first then hit the spoiler-tag. 

Alternatively, you can type out the entire post then highlight the spoiler and click on the eye-icon. The previously highlighted text will appear in the spoiler box. Hit the "submit reply" button and you're done.