I would love to..... If I knew how to do that.
When you're typing your reply, there's a toolbar at the top (bold, italic, etc.). To the right of the emoji icon (the smiling face) is a
⚑ button. You can either click that, or you can use the
••• button to the right of that. If you click that second button, you'll see different tags available; choose "SPOILER" from the list.
In either case, you'll then be prompted to give a name for your spoiler;
this is optional. The tags I indicated before,
[SPOILER]
[/SPOILER]
, will appear where you're typing.
In between the middle two brackets, enter whatever spoilers you're including. So you might have
[SPOILER]
something like this
[/SPOILER]
in your post.
You can also manually enter the tags (type them yourself), again having them on either side of the spoiled information. Finally, you can highlight some of your post and then click one of the options for SPOILER tags that I mentioned before. After choosing (or ignoring) the name for the spoiler, the tags will automatically be applied to either side of the text you've highlighted.