Well, that's the complicated part. For NTSC content, you might use an NTSC shader with a color hack or shadow mask, integer scaling, and either RBG, composite, RF, or S-Video depending on the generation of console or cabinet you're going for. Some people just want to recreate the experience of their childhood TV, others want to model a quirk of a specific tube or board rev. Lots of options!

If you haven't, check the libretro forums for "new-sonkun-crt-guest-advanced-presets-thread". I've only tried with RetroArch for management, but it's not limited to that.