This gets asked alot.. I’d do a search for this.. however here are some quick tips..
Create your Composition with “1 person”,.. is this a still or video?
This will be your “Custom” particle under the Particle Type.
Set your Emitter to a Box emitter, and set the velocity to 0.
Perhaps do an Emitter size of X for 1000 Emitter Size of Y for 0 Emitter Size of Z for 1000.
You’ll have to let the particles generate however you can Pre-Run the Emitter with a percentage under OPTIONS.
Physics Time Factor will stop the particles from generating Keyframe 1 to 0.
If it’s a Looping Video, then I’d keep your video real short and under Time Sampling select Random Loop. This is off the top of my head right now.. but you’ll be able find a lot more detail in your search
“kis” it
Keep it Simple