I like using syringes because it gives you the quickest measuring method and you can directly place it in your bottles. If you use too many steps in the mixing process, you can get confused, make a mistake, and also dilute your measurements the more you transfer liquids from one container to another. I have 1ml, 3ml, and 10ml syringes. Whatever method you choose, just keep with the same one so that you are consistent from recipe to recipe.
I mix on a tray so that if I spill, the nicotine does get on the countertops.
I use gloves as I have 100mg nicotine. I also use goggles if I am working with the nicotine solution.
If I make a mistake and need to discard any nicotine, I don't pour it down the drain so that it doesn't contaminate the water supply. Instead, I use paper towels to soak up the liquid and put that in a zip lock, then toss that in the trash. It's more garbage, but it's safer.
I try not to have too many things going at one time so that I don't lose concentration and make mistakes.