I've found success with the spot almost directly below the very back of my jaw on each side of my neck, it's higher up than my adam's apple.
I've only managed to do it when I've been sitting up so far (still trying to figure out how to do it lying down). I use three fingers on each hand, pushing in vertically (to the neck, you want it to be tall, not wide) on the spots where you feel the pulsing is strongest. For me, it manifests very quickly, starting with light-headedness and going into a very quick black-out of all vision, from the edges of my vision inwards. If you aren't getting promising results in ~5 second, position your fingers slightly differently. In my experience, the spots on either side of your neck might be (very slightly) different, sometimes you may only have to adjust one side if the other feels like it's right. Pay close attention to the feeling in your fingers, it takes some practice but you should be able to start telling if you're hitting the right spots.
I'll try to remember to update if I find anything extra while trying to get it to work lying down. Hope this helps someone.