The simplest way to make natural cough syrup is to boil your herb (or herbs) of choice in water. The resulting liquid is now an herbal tea which can be strained and taken on its own. If you want to make a syrup, however, you'll need to let the strained tea cool just a bit and then add honey to the mixture. The amount of honey you add will vary depending on the amount of tea you have, but the resulting mixture should be thick enough to coat the throat.
Teas are a great way to get the benefits of sage all you need to do is steep one teaspoon of sage in a cup of water for about 10 minutes. You can also make a pretty awesome sore throat reliever by combining sage and thyme. Take an ounce of both grind them, and cover them with 16 ounces of apple cider vinegar. Be sure to shake it periodically and let it sit for ten days before using.