There seem to be two main schools of thought regarding attaching a ref patch to a jersey. If you only work one league, then most officials will sew the patch right onto the jersey. If you are going to be working multiple leagues where the patch would need to be changed, people will sew black and white velcro onto the jersey and then velcro onto the back of the patch.
Regardless of method, (for USA Hockey patches), the patch should be lined up so the bottom of the patch is in line with the bottom of the zipper on the left chest and then centered between the zipper and the seam of the sleeve.