cut the tip off the frosting tube, cut it small, you can always make it bigger

sketch the Jack Skellington face on paper a few times just to get some practice

then try it with the gel on paper to see how it flows, once you've got it then you can go for it on the Ding Dongs

make his eyes first, use a toothpick to pull out the edges a little bit to fill them in if needed

then make an oversized smile

use a toothpick to draw little lines down from the mouth

mirror those lines going up to finish his mouth

use the last bits of gel on the toothpick to make two dots for his nose

repeat with the remaining Ding Dongs