Okay so, you will need either Hamachi or Radmin VPN for this. I prefer using Radmin because I'm more familiar but Hamachi is just the same thing.
ANYWAY, for the server configurations, I suggest you leave them default except for the amount of players but the amount that you'd like is up to you.

So in either Radmin or Hamachi you're gonna make a network.
Make sure the network has a unique name like HL2 with my homies 1294889372 so that it's not taken and give it a simple password like adada or 12345 or password so that your friends can get in without the hassle of typing "jamaljaquontavionjohnathan".

For your friend on the other hand they're gonna need to download the software that you used, being either Radmin or Hamachi. after this they have to click on "Join an existing network" and type in the network name and password.

after they have joined the network give them the ip address that appears on your Radmin or Hamachi. on Radmin the ip address is thrown at your face, it's next to the ON button. same thing with Hamachi.

Okay, now with Hamachi open and your friend in the network, you can finally host the server. after you've hosted, give your friend the IP address so that they can join.

And now for your friend's turn to do work!
Okay, so if you're playing half life 2 multiplayer, you've obviously played a source game before but if for whatever reason you haven't, just go to settings, keyboard, advanced (at the bottom) and enable the developer console.

After this you have to press the ` key on your keyboard (it's near ESC) and type in connect [IPADDRESSHERE] and then enter the command. after that you should be joining and you and your pal should be good to go!

Thank you guys for reading this awesome guide on how to play Half-Life 2 with your friends!