Use that convenient medicine dropper you got from the pharmacy or that came with your baby's medicine to measure out the right dosage. Then take the nipple of a bottle and squirt the medicine into it. Hold it with the hole down so all the medicine pools into the top part of the nipple. If it's easier to hold, attach a bottle to it, when you give it to your baby. I simply hold the nipple and watch as she sucks it all down. I use this "technique," if you will, for vitamins, amoxicillin, tylenol, you can even use it for gas drops.
I've found some medicines, like baby tylenol, are a little too thick to come out the hole in the nipple. Try using a fast flow nipple that has three or four holes (usually for older babies 6+ months). The other option is to sacrifice a smaller nipple and use a sterilized pin to make the hole slightly larger.