You will need:
2 cups sugar
1 cup water
1 tablespoon vanilla extract
a healthy supply of seltzer (carbonated water), half and half, and cubed or crushed ice
Step 1: Make syrup.
Put 2 cups of sugar and 1 cup of water into a small saucepan. Put it on the stove and crank the heat up to high. Stir it up until all of the sugar is dissolved. It will look cloudy, but let the solution come to a boil and it will automagically turn clear. Once it's clear take it off the heat and let it cool for about 5 minutes.
Step 2: Add vanilla.
Now use a funnel to pour your syrup into a storage container of some sort. A plastic salad dressing bottle works well since you'll need to be able to pour the syrup. Make sure the container is big enough. The one I use is 600 ml.
Store it in your refrigerator.
You don't want it to melt the ice in your soda, so it's best to chill it before use. I like to give it an ice bath if I want it for immediate use.
Step 3: Mix it up. Enjoy.
Stir it up and you're ready to enjoy!
Alternatively, you could leave the vanilla extract out of the syrup and use it to sweeten other soda concoctions. Try, for example, plain syrup with the juice of one lime.