In these documents you will find information for a specific subject(World geometry, scripting et cetera...), I recommend following along with the tutorial.
Serious Editor is a powerful tool for every Serious Sam modder, with it you can create levels, models, enemies... The editor itself may be intimidating at first, but once you get a hang of it you realize it crashes a lot(haha...). Nevertheless, it really is satisfying once you start creating with it. We'll be creating some small levels to show certain features of the Serious Editor. Of course, you are encouraged to add onto those levels and try some stuff yourself, don't be shy to send your fellow friends your maps (send some to me!)
Here are some screenshots that show what you can do with Serious Editor!
Yes, the third image from left does indeed show a pillar with rocket launchers.
The fourth screenshot is from a map called "Lost Forest", created by Noam2000, you can find it here, that's the Steam Workshop link.