I have my kids (age 9 and 6) scratch the car first and then sand the decals with 150 grit paper. Then I give them a dry brush of various browns, greys and blacks (a little tiny squirt of cheap acrylic paint from the craft shop on a paper plate - dab a little with the brush then brush it out on a piece of newspaper until almost no paint is coming off). We concentrate the black along the bottom of the car to mimick the grease and grime.
For the patchy rust on the hopper below - I took a piece of plane white paper, folded it to mask the whole car and then tore out the rough edged shapes from the top corner down towards the wheels. I then sprayed rust colored spray paint from a distance. I think that worked great too and it took less than 3 minutes to do it.
Otherwise, practice, make mistakes, see what you like and look at lots of other people's work or at the web. O good search on ebay is "weathered ho". There are some real artists selling their work here.
See ya, enjoy.