I don't get FOFs.

I read the wiki, got the sample wad... but I still don't understand how to make FOFs. Could someone help?

I can help you.
You must make a control sector (a triangle 32 x 32 x 45) OUT of the thok barrier.
Then you pick one of the linedefs of this triangle and set like this:
*The FOF's sides texture
*The 100 Linedef Type
*The tag 1
Then, go to the sector (triangle) and put like:
Ceiling height: Height of the top of the FOF
Floor height: Height of the bottom of the FOF.
Then, you select the in-map sector that will be the FOF and give it the tag 1.
Voilà.

I did that. The second the game started up, I died.

Why? You did something wrong. Can you send me a copy of your wad, Virt?

Ohh, nevermind. I only set the tag for 1 on the sector, not the linedef. Thanks, Kim!

You're welcome.