OK. I have attached this to my Singer heavy Duty 4423 machine by following the supplier's instructions. First you remove your snap-on foot, then you unscrew the bit that houses the snap-on foot because all this is, is like a screw-on foot. You are left with the bare shank with a hole in it. You put the white plastic fork onto the needle bar (this is what makes the feed dogs on the foot go up and down along with the needle. Then screw the whole thing to the shank, like a screw-on foot. Tighten both the needle screw and the foot screw firmly. My needle dropped out of its housing because I didn't.
Then get sewing and be amazed at the result.
I seem to recall someone saying in the questions that you can't use this with a snap-on foot machine. You can if the snap-on foot thingy is one of the 2 types 60c (the manufacturer) say it should be. i.e. a removable one, I assume.