Start off by laying the jersey on your lap or on some flat surface. Position the number where it needs to go and push the fist pin in about half way through the number and jersey.
The pin should go through the jersey and back out through the number. Kind of like sewing the number to the jersey with one stitch of the pin.
It should come out like this. lock the pin and you've got 1 down, 7 to go. Don't use the holes on the corners.