Creating Spoilers

The most efficient way to insert a spoiler is to use the button with eye icon from the tool-bar at the top of the editor.

Clicking this icon will insert a spoiler box into your post at the current position of your text cursor.

Spoilers within spoilers also are possible by using that button, simply click the button while having the text cursor inside the spoiler. For example, double click it to create a spoiler with a spoiler inside.
 

You can also create a spoiler using BBCode:

[spoiler]This will be in the spoiler.
That too.
Yeah....[/spoiler]

This will work in a small scale, but weird things happen when the post gets big. Generally I do not recommend using BBCode tags when wanting to make spoilers.

Tips and Bugs

 If you'll hover your cursor over a spoiler, you'll notice arrows in the upper left corner of the spoiler.

You can click&drag that to move spoilers around. It is possible to move these in/out -to/of other spoiler, but be careful, as they sometimes get eaten along with their contents when moving spoilers within spoilers. If that will happen, you can try to undo the operation, by pressing CTRL+Z.

 When moving spoilers around (it tends to happen randomly as well), it is possible to move these to the very bottom or the very top, so that you can't type above or below them, due to lack of space.

The only solution is to drag the spoiler up/down a little, so that you'll have at least one free line. This sounds obvious, but it can be extremely frustrating for users, who aren't aware of that.

 A safe way to remove the spoilers along with their contents is not highlighting them, but clicking their top and hitting delete key. Backspace will also work.

 Selecting special boxes (such as spoilers) may be a hassle.
Let's say, that you'd have some text with a spoiler in the middle. Now you want to select it all in order to copy&paste it somewhere else. If you'll start highlighting from the top, everything will work nicely. However, if you'll be highlighting from the bottom, your selection will get stuck once you'll hover over the spoiler:

A solution is to go around these boxes when highlighting. Just move your cursor to the left, then drag the selection up.