Ewing,
To put on the makeup, start with clean skin - washed and dried, no antiperspirant or anything on top. That'll help the makeup or temp tattoo stick the best. For the makeup, apply with a small brush, or you can look for the body paint as a crayon, where you draw it on. Have a reference of the full design to look at, preferably at the size you want it (so, for a 4" diameter flower, find a reference pic and draw out the whole design at 4" on a piece of paper). Since this is on your neck, you'll probably need someone to help you draw on at least part of it if not all. Drawing in the mirror is VERY hard to do right.
Once it's drawn, lightly dust some setting powder on with a big poofy blush brush (can find at drugstores). The idea with the powder is to absorb some of the moisture in the paint and help it dry faster. Do this lightly, so you don't smear the paint with the brush. The paint will never be "dry" enough" to not smear if you rub it with your fingers/shirt/etc. though. The setting spray will also protect it from smearing, and also from sweating to a certain extent, but it's not a perfect barrier. Be very mindful of your shirt rubbing on the tattoo. Might be good to bring a Tide pen with you so you can rub off any makeup marks on the shirt collar after the first day.
You're right, this won't last for more than a day. Wash it off before you go to sleep, otherwise you'll wake up with paint in other places (face, hands, sheets, etc). The temp tattoo would probably last through the night but might need some touching up in the morning (you know how temp tattoos eventually start cracking and little bits flake off). I don't think the setting powder would do anything for the temp tattoo, but the setting spray might give it some extra staying power. Overall the temp tattoo might require less maintenance and fussing, but it's harder to apply large tattoos to heavily contoured areas like your collarbone. I've noticed that really large temp tattoos tend to look kind of shiny, like a patch of painted saran wrap.