Step 1: Add the carbonation to make carbonated water
You can add carbon dioxide to water by using a soda kef or cornelius keg. You will also need a bottle of carbon dioxide and a regulator to meter it out... 50 psi worked OK for mine, but you may want to experiment to see the amount of bubbles you need. Here's a picture of the soda keg:
Source: Wikipedia Image
If you don't want to put up the money to buy a full soda keg setup (they currently run a couple to a few hundred), you can also buy seltzer water without any added salt. Check the label and make sure it *only* has carbonated water.
Step 2: Add some flavor!
To give the soda some flavor, you will next need to add a flavor base. Companies like Sodastream sell pre-made flavor packs, but you can also try these ingredients to make your own, all-natural base:
- Lemon-lime: Lemon and lime juice
- Root beer: Sassafras tea, vanilla
- Citrus (like Mountain Dew): Orange juice, lemon juice, optional ginger
- Cream Soda: Vanilla
- Fruity Soda: Concentrated juice
For all of these, you will need to add sugar. You might be surprised how much you need to add!