i basically do them with a shoulder-wide or maybe a bit wider grip, the bar is on a level with my throat or clavicula
as opposed to skull crushers my upper arms don't point straight (or as straight as i can keep them) in the air, but are more extended to the sides, making the jms look like a hybrid between close grips and skullcrushers
i don't know if i do them right but i really feel them in my tricpes and chest (which i think is because the pectoralis has a hard time stabilizing the weight because of the flared out arms)
watch out when going heavy for under 5 reps on them, when i'm shooting for a best of 3 i tend to bring the bar down on my upper chest, doing more of a shoulder-close-grip-exercise...
there is also a variation on them, i forgot the exact name, when you got the bar down at the height of your clavicula you move the bar a bit towards your chin and then push it up
i tried it out once and got crushed, it's much more difficult this way
hope i could help