It’s pretty straighforward from the motionscript website how to do it, and Dan in the expression forum here is the guy who actually made the webpage and more than likely wouldn’t mind helping you tweak your expressions up a bit.
There’s like a million posts on here you can search out but the best effects I have found are cc vector blur and particular, they are simply amazing plugins. Videocopilot has a bunch of fire videos too that focus on details like small particles and smoke. Those sell a lot, but the most important thing to remember is that both the intensity AND the interval have to be random.
https://technicolorsoftware.hostzi.com/