I have the Designer 1 but I would think it would work the same. Raise your presser foot. Unscrew the ankle that holds on your foot.
On the right side of the foot as it faces you there is a bar with a "U" shaped piece at the end. That "U" must fit over the needle bar (where you loosen the screw to change your needles).
Once you have that over the needle bar, line up the two pronged 'ankle' with the presser foot bar and screw it on.
Be careful, try to hand screw it in. I used my screwdriver once and accidentally started cross threading the screw in area and now I have to be very careful to line the screw up 'just so' in order to screw it on.
Hope this helps. There should have been a little instruction sheet with the foot or on the back of the card (if the foot was carded).