since i couldn't find the right striped fabric, i just made my own. but maybe you will get lucky and find some :) you will also need a sewing machine, needle, thread, scissors, and velcro.
step 1: cut a section about 1" wider than you want your bow tie width and about 4" longer than your bow tie length. then iron in each side like the picture above.
step 2: pinch the middle to form a bow shape.
step 3: insert the needle and thread and stitch a few passes to keep the bow shape. leave the needle attached. cut a small section for the middle cover and a long enough length to fit your neck. you can also you ribbon if you want to skip adding the velcro.
step 4: place the neck piece on the back of the bow and wrap the small middle cover around both. stitch the small piece tight.
step 5: sew on two pieces of velcro on either end of you neck length piece, making sure that the velcro matches up on once around your neck.
done!! enjoy wearing your fancy harry potter bow tie :)