As a IICRC Certified Carpet Cleaning Technician, I can tell you will absolute certainty the proper process.
First, do NOT use a foam product. The problem with foam is that a soil-attracting residue is left in the matts, which means that while they may be clean as soon as you're done, they will not be clean a week or two later - dirt actually "sticks" to the foam and cannot be vacuumed out.
When it comes to cleaning matts, I would suggest the following (extremely cheep) way of doing it.
1) Fill up a square plastic tub (like one you would store winter clothes in) with water and an upholstery specific cleaner (use dillution ratio provided on the label)
2) Drop the matts into the solution and let sit until fully wet
3) For problem areas - scrub with either an upholstery brush or a soft-bristle brush
4) Remove matts from tub (also called an "immersion bath")
5) Treat any remaining stains with 303 fabric and vinyl cleaner (undilluted)
6) Rinse with a garden hose, pressure washer, or whatever you have
7) Apply a SOLVENT-based protectant, such as 303 High-Tech Fabric Shield or Chemspec's All Fabric Stain Shield
Scotchgard only protects against dirt and debris - a solvent based protectant protects against everything!
Hope that helps