When I first started using 100mg, I wore gloves. But I don't do that anymore; I'm actually clumsier with gloves than without, and even if you get a bit on your fingers, it's not a catastrophe; just rinse well with cold water (warm water opens your pores and nicotine will absorb very easily into skin, even better with opened pores!). Both PG and VG (the common carriers of nicotine) are water soluble even if they *feel* greasy, so cold water will rinse the PG or VG + nicotine very well.
I usually place a paper towel under the bottles when I'm decanting the 100mg from the 120ml plastic bottle it came in, to the 30ml brown glass bottles in which I freeze it; after the bottles are all tightly capped, I give them a cold-water rinse and dry with paper towels before affixing the labels. The only messy-factor I've found is that the plastic bottle I'm pouring *from* tends to drip down its side, so if it makes you really nervous, gloves aren't a bad idea. Just don't have any pets around while doing this; pets are often prone to licking up anything that falls or drips to the floor, and you certainly don't want them licking up any 100mg nicotine.
Just mixing
ejuice from one of those 30ml glass bottles, I don't worry about it; I use a syringe to pull up the nicotine, so there's really no messiness involved.
The main thing is this: you don't want to drink, or even vape, 100mg nicotine, because yes, it is toxic. But it's not nuclear waste; getting a few drops on your fingers will not kill you, just rinse it off. Mainly you just want to be sure not to leave any drips around that a pet could get to. I'm assuming that you don't let any small children lick up your countertops -- I could even go so far as to assume that you don't let your small children rampage around the kitchen unsupervised, but who knows, these days.
Andria
ETA: it should go without saying, try to not get your face really close to the 100mg -- you don't want to accidentally splatter any in your eyes; it will burn like acid and absorb into your bloodstream almost instantly. I once rubbed my eye forgetting that I had just refilled my RDA, and even a trace of 10mg
ejuice burns the eyes like acid!