I don't often fit others, but I need to fit my bridesmaids' outfits. I have the Craftsy Class 'Sew the Perfect Fit' with Linda Maynard. She fits toiles (or muslins as she calls them) by folding up the fabric on the model, or cutting it and pinning in additional strips of fabric. In Susan Khalje's Couture Dressmaking Craftsy class, she pins out little tucks of fabric, but her model requires very few adjustments. Both seem to just do it by eye, presumably as a result of experience, but both work from the top down.
I wonder how else people fit toiles? Have your methods changed as you built up your knowledge and experience? What reference materials do you use, or did you use when first starting out? Is working from the top down an established method? My instinct would be to start with the biggest problem area, but this is probably wrong!
I have a couple of books (Vogue and Winifred Alrich's pattern drafting one) which illustrate different fit issues and how to fix them, so I suppose one way would be to look up each issue and read up on how to fix it. I could then use the adding or removing fabric methods above to implement these. Presumably, the more often I fix an issue, the less I will need to refer to the book for it in future!