Begin with studying your fashion illustration textbook. This book ideally will teach you how to convey your designs on paper and simulate the look of the fabrics you want for your designs.
Study your flat pattern textbook. Your flat pattern textbook should ideally show you how to make a set of slopers, which are garment piece templates for creating patterns with. Then the book should teach you how to make patterns for basic garment such as shirts, pants, skirts, and dresses.
After you have practiced enough creating patterns for simple garments begin experimenting with making original patterns for your designs using your textbook as a guide. This takes time and patience so be prepared.
Study your draping textbook. Use your dressform and muslin to practice draping designs that will later be transferred to paper. Purchase any additional supplies the textbooks indicates you may need in order to become proficient in your draping skills.
After you have become comfortable with your flat pattern and draping skills. Begin studying your sewing textbook to learn different sewing techniques. Use your muslin to practice sewing each technique listed in your text on your sewing machine and serger.
Once you have become proficient in sewing, lay your practice patterns on your muslin and cut the them out using your dressmakers shears. For each seam refer to your sewing book to find out how to join your garment pieces together. This skill takes time as well so be patient and keep trying until you get it right.
Once you feel comfortable with your design, flat pattern, draping and sewing skills practice creating your designs from start to finish in the fabrics of your choosing. Experiment and try new things to fit your style.