So, for the 90s photo effect, what you want to do is pretty much go into Photoshop.
Once there, you can go to IMAGE -> Adjustments and click on COLOR BALANCE & SELECTIVE COLOR.
This is what really give it that 90s effect. Go into those and alter the colors to your liking.
It really helps if you have an example of an old photo. I’ve seen plenty, so I have an idea.
Make it look dull, you can go back to IMAGE -> EXPOSURE and alter the brightness and stuff there.
Then I go to FILTER -> NOISE and click on ADD NOISE. I like a lot of noise, but I’ve never really maxed it out. I usually use around 100% (that really isn’t a lot, don’t worry).
Then immediately after adding the noise it’ll look terrible, so you go to EDIT and click on FADE NOISE. Once there you can fade the noise and also mess with the effect (i.e. normal, soft light, hard light, etc.).
Then I go back to FILTER and to BLUR -> FIELD BLUR and blur it out to 2px.
After you get the colors you want, mess with exposure, add the noise, and the blur, you should get results similar to mine.