It all starts with an idea or inspiration and you start collecting inspirational pictures, or items that fuel your creativity for a new collection. You create mood-boards, inspiration-boards and any board that helps you put together your vision for your next creations. You research trends, source fabrics and notions that work with the idea/inspiration.
Than you make a ton of rough sketches and explore ideas in a sketch book, chose style lines, fabrics and silhouettes, until you come up with a cohesive rough collection.
Than you start drafting a pattern for your first "muslin"
(When sewing clothing, a dressmaker may test the fit of a garment, using an inexpensive muslin fabric before cutting the intended expensive fabric, thereby avoiding potential costly mistakes. The muslin garment is often called a "muslin", and the process is called "making a muslin". With the availability of inexpensive synthetic fabrics, which closely resemble the hand (drape and feel) of expensive natural fabrics, a test or fitting garment made of synthetics may still be referred to as a muslin, because the word has become the generic term for a test or fitting garment.)
After the fitting of the muslin you adjust the pattern with necessary changes and add hems, collars and all the finishings, than you start cutting the final fabric and sew.
Once the garments are finished, you organize a professional photoshoot and create a look book.
You create marketing material and invites for a fashion show.