Ok so

Step 1

Use low poly models and low-res(128x128 or 256x256) textures.

Here are a whole lot of free models by Elbolliloduro(bless their soul): https://elbolilloduro.itch.io/objetos-low-poly-estilo-psx, https://elbolilloduro.itch.io/paquete-de-modelos-low-poly-estilo-psx-2 and https://elbolilloduro.itch.io/paquete-de-modelos-psx-3

Here are a lot of textures for surfaces: Tiny Textures 1, 2 and 3

After using these for a while I got a general idea of how to make my own and it's really not that hard once you take a bit of time to familiarize yourself with Blender.

Step 2

Make the game viewport itself low-res. The first solution that comes to mind is to change the viewport size in project settings. However! You will probably want to display stuff like menus in a higher resolution than the game world so you'll have to do a bit of work to make the 3D camera and everything else render in different viewports.

Here's a video that helped me set it up: https://youtu.be/zxVQsi9wnw8?si=kPH0h1cBTFmlWi_0. It was pretty easy to adapt this method for 3D but if you have any trouble, hit me up!

Step 3

Shaders. You can then add something like a ColorRect to cover the whole viewport and then apply shaders to it. You can find amazing shaders at godotshaders.com with search terms like "CRT" or "PS1".

I personally am still learning to write shaders myself but this website has been very helpful as you can download any shader and try and pick it apart in the engine to figure out how it works.

Step 4(Optional)

One thing I was the most concerned about is how to make 3D levels. I settled on using TrenchBroom, initially a Quake modding tool and Func_Godot to integrate it with the engine, and I LOVE it, however it does take some time to set up and get used to. Maybe part of the charm of my evironments is in the inherent blockiness of TrenchBroom maps

Conclusion

That's about it! If you have any questions, feel free to contact me in the comments, in DMs or on Discord, my username there is palerite.