So.... This post sort of goes against my feelings about Hex Oil. We usually say that we'll supply the means but we won't tell you how to use it. I believe that if you're going to get into dark magic, you should do it on your own - or without the alchemy of someone who isn't involved.
With that in mind, here are some ways I would personally use the Hex Oil. I'd first start with an incantation, it can be as rhyme-y and intricate, or as basic as you'd like. Then, you can do any of the following...
Put some drops in their path, at their work station, on their pillow, etc. Anywhere they are going to be, so they can get the oil on themselves - preferably directly on their skin or hair
Burn or bury - but don't bathe or carry. You don't want the spell to affect you or backfire
Put some on your hand and shake theirs, or touch the oil to them somehow. Again - bonus points if you get it directly on their skin or hair
Write their name on a piece of paper and soak it with the oil. Burn afterwards, if possible.
Let us know if you come up with any other methods, or have any great success stories to share!