If it's a pill, push it into a piece of hot dog or cheese, a spoonful of peanut butter, or any treat you know your dog enjoys. The trick is not to put the pill in the first bite of food but to hide it in a subsequent one. This will prevent your dog from suspecting something is in all her treats and avoiding them altogether.
If the drug is a liquid, pull out the side of your dog's cheek and, with a syringe or teaspoon, squeeze or pour the medicine into her mouth. Avoid tilting her head back or forceful squirting, which can cause choking. Reward her with praise or a treat.
As an alternative, you can try pushing the pill into your dog mouth, beyond the base of the tongue, and holding her head high until she swallows. Follow up with a reward. This method works best for pets on highly restricted diets that cannot or will not take medication with a treat.
For a demonstration, watch this video: