You, as Morgan, designed a supercharged Nullwave prototype that is capable of knocking out all of the typhons in and around Talos 1. Manufacturing, placing and triggering this device will bring you to the end of the game.
This is the least destructive ending, so we guess that makes it the "good" ending. It does kind of assume that all future containment protocols will be successful, so maybe this is better described as the "optimistic" ending.
Get Nullwave Device plan
You already have the plan — Alex gave it to you while he explained himself at the end of "The Keys to the Kingdom."
Fabricate the prototype Nullwave Device
Get to a fabricator — there’s the one at the GUTS entrance to the Arboretum if you’re in a hurry — and make it.
Place the prototype Nullwave Device
You can’t just put the Nullwave Device just anywhere. Alex will call when you have it in hand and tell you to place it at the "Heart of the Coral."
The Heart of the Coral is on the satellite you encountered (saw, but probably didn’t get close to) during "Detour." Head to the Talos 1 Lobby, then into Psychotronics. The Apex Typhon has its tendrils everywhere, so you’re going to have to be on your toes — the shotgun works wonders against the tendrils, by the way.
Head all the way back through Psychotronics, through the security checkpoint and the Clean Room where you picked up the psychoscope. Head through the decontamination airlock and take the grav lift up to the Psychotronics atrium.
The atrium has some tendrils, a phantom and a weaver waiting for you. If you’ve got the ammo, the Q-beam will tip this fight in your favor. Regardless of how you handle it, it’s in your interest to end this fight fast — you’re starting off outnumbered and it’s only going to get worse — so don’t be stingy with your charges or ammo.
Continue past Labs A and B, then turn into either the Materials Extraction or the Live Exam room. You’re not doing anything in these rooms. You’re just trying to get to the grav lock on the other side of the chamber. It’s the same room where you found your way into GUTS the first time.
Activate the prototype Nullwave Device
Once the Nullwave Transmitter is in place, Alex will call and invite you up to the bridge to trigger it. Go up to the Arboretum, and then make your way to the Talos 1 Bridge.
Hear out January and Alex while they make their cases. January has the console locked down, so if you’re committed to this ending, you’re going to have to destroy January — doing this will get you the "Suicide by Proxy" achievement/trophy.
Hit the button on the console to trigger the end of the game. (See "A Hidden Place" for what happens next.)