It's quite simple
First of all, you need to click on a button above the text field. The button it's a speech balloon with the word 'Spoil' on it. Very easy to find.
Then press that button and select the title for your Spoiler. For instance if you're selling items, you can use as one of the titles: 'Forces'.
Then you just have to write on the middle of the two commands, like this: [.SPOILER="Forces"]blablablabla[/SPOILER].
Here's the example of how it looks:
Marine force - xxxx TC
Aqua relax - xxxx TC
etc.
...