First of all get the HPL2 Mod Programs here, it'll give you instructions to do that on the page:
https://wiki.frictionalgames.com/hpl2/tools/start
Then after that read the 'How to Start' area of this page until Step 4, the instructions might seem trivial but they DO help, trust me!
https://wiki.frictionalgames.com/hpl2/start
After you finished Step 4 I would recommend checking out
Mudbill's tutorial series. He's very smart and a cool guy in general. And dat accent thou.
Then start making maps! Make one map first to warm yourself up, don't aim too high like 72% of first timers with HPL2. Be that first timer who's like "Yeah, I'm the guy who actually finished a working map that actually has gameplay and tension without having to do 500 other maps just to warm myself up"! Be that guy/girl!
Good luck and fáilte to the Frictional Games Forums Community, enjoy your stay.