As Defcon mentioned in the spoiler rules thread - AntonyF has implemented a "Spoilers" prefix that you can add to your thread title. (Thank you, Defcon! I haven't been keeping up in there
)
When you click on Create New Thread, on the left of the bar where you write your thread title is a drop down list of prefixes that you can select (or just leave with no prefix). One of these is Spoilers.
(In the old gray colour scheme, this appears as an empty pale gray rectangle until you peer really closely to see the tiny down arrow indicating a drop down list)
It's not mandatory to use it, but it's useful if you want to.

When you click on Create New Thread, on the left of the bar where you write your thread title is a drop down list of prefixes that you can select (or just leave with no prefix). One of these is Spoilers.
(In the old gray colour scheme, this appears as an empty pale gray rectangle until you peer really closely to see the tiny down arrow indicating a drop down list)
It's not mandatory to use it, but it's useful if you want to.