I make Big Ass Bindis ( www.BigAssBindis.etsy.com ) I can give you a few suggestions:

If you are using rhinestones purchased from the craft store, they are made of plastic or acrylic and they are mirrored (silver backing) in different ways depending on the quality of the rhinestone. E600 can disolve the silver backing and give the rhinestone a cloudy, crackled look. Not something you have to worry about if you use glass stones, but glass stones are significantly more expensive and aren't as easy to find in stores. I use 4 different adhesives, what you are glueing and what you are sticking it to is what determines what type of adhesive to use. Experiment with stuff to see what holds and doesn't damage your stones or your backing.

Use Spirit Gum, not eyelash adhesive to glue them onto your skin. Eyelash glue works fine to re-use the sticker type bindis from India because they are so light-weight. Handmade bindis tend to be heavier and spirit gum can handle the weight. Olive oil on a cotton ball will wipe the spirit gum residue off of your skin without having to pick at it or rub hard. Besides it's something most of us have in the kitchen anyway. (I remove my eye make-up with it too).

>> how do you get all the gunk off of the back of it?

The same way I remove my eye makeup -- olive oil. A little bit on a cotton ball or Q-tip does wonders. It won't dissolve the backing of your bindi or irritate your eyes/skin. Also removes eyelash adhesive & Eye Kandy like a charm. It's a glorious thing. Seriously.

If the bindi backing is made of acetate, you may have a problem with spirit gum. But if its vinyl, it'll be fine. Bellatrix is right on the money with the olive oil to clean the backs, after you've wiped all of the skin/make-up/spirit gum mess off of the vinyl back, dip a Q-tip in rubbing alcohol and give the vinyl back a wipe. That will clear any oily residue so that you won't have a problem getting it to stick on next time.

I put flat-backed gems directly on my skin after using foundation/powder. I haven't really had a problem with them coming off. I do use a fixing spray (the one by MAC), so that could make a difference. I just use some eyelash glue to put them on.

X-acto works OK, but I find applique scissors work best. I suppose manicure scissors would work well too, although I've never tried it personally.

When I first started I used an exacto knife, but I found that it was difficult to get a smooth edge around the rhinestones. This is likely because the backing I use is thicker than what Sandi uses, for example. I quickly switched to regular scissors, which work fine.