2D Backgrounds and Enable 3D/2D Mode

I was making a new level and was making a bridge section, which i wanted it to be a 2D section, and I wanted the the wall to be the skybox background, can any one help me? I'm also having trouble enabling the 2D mode in a 3D level and the Wiki page is very vague.

What's vague about the wiki page exactly? The Enable/Disable 2D Mode linedef is a linedef executor special, so it'll only work if you made some sector the player enters the 2D mode section through a trigger sector to trigger a linedef executor with, which in turn triggers the switch to 2D mode.

It may be a good idea to check out the example WAD if that still confuses you.

EDIT: You should probably check this out too if you don't understand linedef executors in general yet: https://wiki.srb2.org/wiki/Linedef_executor_tutorial