I have a soiled fabric sofa. What's the best way to clean?

Do a strong wet-dry vac first. Brush with a microfiber towel and vacuum again.

If you don’t get the dust out first you could be making mud.

Then try just wiping with a clean, damp microfiber towel.

I personally try to avoid products on fabric. Seems to me they’re never the same texture. You can try microfiber dampened with plain old alcohol as well, having a clean drying rag on hand.

Oxy Clean, or any other laundry detergent designed for whites should do the trick.

If the sofa is a Bright Bright White, just use bleach and hot water on the entire.

Follow that up with Woolite Spray: Woolite it doesn't do just carpets.