First step: Take a look at the real garment. Try to understand construction.
Second step: Build the garment. Draw your own knitwear structure in Adobe illustrator and put into the program. Add elastic fabric effect, fur pile and some details.
Third step: put light settings everywere. Make background of the video in Photoshop. Create animation for the model.
Last step: Look at the all render videos and create one overall in Vegas pro.