During my studies at the Academy of Art University, I have been focusing on improving my sketching and rendering skills. It is challenging but truly exciting - fashion illustration is ultimately a set of rules that can be learned. I have enjoyed reading books and watching tutorials on how to properly draw the fashion figure and render fabrics. A page from my sketchbook below shows color illustrations I found on Pinterest and my black-and-white sketches of these illustrations. Copying existing illustrations is a good exercise for hand-eye coordination and it is a fun activity for a rainy afternoon ...
If you are getting into the field or just want to improve your own style, many artists share their knowledge on YouTube. I find tutorials by Yelen Aye and Zyra Bañez particularly applicable to what I need to focus on. Check them out!