Is there a tutorial how to add custom roads?
No, but I'll explain it for you.
Download any road mods, it'll usually give you a .rar package, place it in any folder and extract it. It'll give you many files.
After that, download Alci's IMG editor (https://www.gta-modding.com/area/file-3-alcis-img-editor.html) and open up the program (make sure GTA SA/MTA SA are closed) and then click the button at the left top side of the program, and open your gta3.img file, it's inside your GTA SA folder inside another folder called "models".
Then, click on the green arrow pointing down, which says "import" and then import all the files that you have extracted before (the road mod files). Now, just click on the "save" icon and then it's done. @OneOfAngels
And here's a video if it's still not clear enough.
Show content
https://www.youtube.com/watch?v=-m2UgZAEJaI