As Monday’s Met Gala approaches, the art of fashion is top of mind — and celebrity stylist Erin Walsh is breaking down her process. On Thursday night, the stylist hosted a cocktail party at The Fifth Avenue Hotel’s Portrait Bar in celebration of her new book, “The Art of Intentional Dressing.” Anne Hathaway , who penned the book’s introduction, served as a cohost for the evening. The actress, on the eve of the official theatrical release of “The Devil Wears Prada 2,” was dressed in a black crepe top and trousers by Stella McCartney for the occasion. Other guests included actresses Ariana DeBose and Beanie Feldstein — who are both styled by Walsh — as well as designers Stella McCartney, Prabal Gurung , Fernando Garcia, Jennifer Fisher, Fletcher Kasell and Tanner Richie. Stella McCartney, Anne Hathaway , Erin Walsh and Wendy Finerman.…