I have no idea how to write a guide, but I feel the need to write this because I found this solution under a reddit comment chain so people might possibly miss this. Of course credit to u/battlegoat42 and u/ISSAvenger for the method (https://www.reddit.com/r/Starfield/comments/169jzir/anyone_running_43_or_54_resolution/), I am just the messenger.

The way you can use custom resolution in Starfield is to edit either StarfieldCustom.ini or StarfieldPrefs.ini (Personally, mine works with only editing StarfieldCustom.ini. As usual, you can find both of these files in your Documents folder -> My Games -> Starfield.

Edit the file by adding fNarrowAspectLimit and fWideAspectLimit in to what your desired aspect ratio (use 1.25 for 5:4 and 1.3334 for 4:3). Then, add isizeH and isizeW according to your desired resolution. The end result should be like this, this is my actual copypasted StarfieldCustom.ini:


Of course this only works in borderless/full screen, I have no idea how to enable it for windowed mode. But, this is way better than having letterbox 24/7.