extending

I've looked in the forum but i couldn't find a thread or a comment about this, so please if you know about one that already exists, please refer me to it. The thing is that I wanted to learn a bit more about modding, but i can't find a tutorial about HOW to extend pieces. I've seen people...

I'm pretty new to modding. I've only had experience with the Florian mod. I want to get into making puzzles out of other puzzles, which usually involves filling, extending, and a lot of sanding (not in that order haha). I understand everything except the extending. What do you use for it? Why do...