Welcome to Serious Engine!
Later, Croteam developed more advanced game engines: Serious Engine 2, Serious Engine 3, Serious Engine 3.5, Serious Engine 4, and the last one, Serious Engine 2017 - a branch which is supposed to be kept being updated forever.
The main feature of the engine is a good processing of huge open areas, as well as incredible performance. The engine is also popular among Serious Sam gamers, who tend to create new levels and even campaigns on it. For that purpose they use a special tool - Serious Editor.
- See the main page: Serious Editor
Serious Editor is actually a toolkit rather than just a tool - it has a number of various editors inside it. Unlike most modern edtitors, Serious Editor is pretty self-sufficient and does not necessarily requires 3rd party software usage. It has tools for editing models, animations, collisions, particle effects, materials, destructions and much more. That does not bound modders to using just Serious Editor - 3rd party software is welcome too, importing is supported.
When modder's ideas are above default Serious Editor's tools, he can use expansive scripting system. Serious Engine uses Lua programming language for its scripts. It is even possible to make scripts for editor itself, in order to ease the workflow.
Where to start?
Learning Serious Editor is a complex task, and it might take years to master it. However, there are options on where to start, which might differ the result.
One option to start is to meet how work is done in Serious Editor, by trying out first work tutorial. After that is done, you'll have to make an idea of what to work on next. And since then, seek wiki tutorials which will help you out.
Another option is to check Serious Editor video tutorials by community's modder Mauritsio. These videos cover the basics, importing of custom textures, models and music. Teaches you all about cutscenes, particle effects, water and more! One of the best places to start learning.
Wish you luck!
On a side note, there are different Serious Sam communities where Serious Sam fans gather around. And of course, they have modders there as well. You might want to join one/some of these communities to have a place to seek help.