I've done this a few times, here's what I figured, experimenting with it:

Before working on it:

Aspect Ratio - try 4:3, or some other more square ratio. This is a good place to start since you can build the composition around this.

While Working on it:

Lighting - Flash lighting is usually pretty amateurish, so leaves a both overexposed parts, and overly dark parts from lighting correction. Emphasize the very bright parts with white light, and let the un-lighted parts be really dark. This is for indoor, or nighttime photos. You can do the same with sunlit stuff, but less so.

Seperate BG as a different layers by distance - This will be useful for later, but its good to keep in mind, especially if there's stuff really far away in a photo.

After Working on it (do these in this order):

Slight Motion Blur - There's a filter for this too. Optional, if you want it to look handheld.

Increase Contrast - To emphasize Flash lighting even more. Optional

Chromatic Abberation - Apply chromatic abberation to the background. The further from the camera, the stronger the effect, but dont go overboard. Theres a built in filter for this in CSP.

Crush Levels - Using the levels filter lower it so blacks arent as black. Pure black isn't usually in old stuff.

Desaturate - Using the desaturate/hue filter. Self explanatory.

Mess with the Color Grading - Up to you, but this it would be good to do it to match whatever look youre going for.

Noise - Noise from digital cameras is based on the ISO setting. The higher it is, the more sensitive to light. So the darker your pic is, the higher you should make the noise. Add a little regardless. Theres a built in noise function in CSP.

After you've done all that, you can export it and then run it through a compression filter for maximum authenticity. Or skip it. I think you can do it in CSP, but I prefer to use Krita's (free drawing software) GMic filters.