You'll need:
--1c sugar (or less)
--Champagne yeast
--small piece of ginger root
--a lemon
--a grater (microplane or fine holes) or food processor/chopper
--an empty 2L plastic (important) bottle
--around 2L of fresh water
Peel and finely grate a small piece of ginger. How much is up to you; it depends on how strong you want the final ginger ale to be. Try 1T or so in the beginning, and if you want it stronger, add another half tablespoon; weaker, add less.
Add a full lemon's worth of juice to this. If, like me, you don't have a juicer, roll the lemon between your hand and a hard surface a few times; this will help you get the juice out more quickly once it's cut.
To the empty 2L bottle, add:
--1c sugar (Again, taste comes into play. I like it less sweet, so I add ~ 3/4c. You may wish to add more or less depending on your personal taste. Add a full cup the first time you make it, and go from there).
--1/4 tsp yeast
--lemon juice
--grated ginger
Fill your 2L bottle to an inch or two below the top with fresh water. Screw the cap on tightly, and shake until everything is dispersed. Be certain to flush the sugar from the nooks at the bottom of the bottle.
Once you're finished, place the bottle in a warm-ish place (fridge, towards the back) and let it steep. Check on it every few hours by squeezing the bottle. When you can no longer push in on the bottle (i.e., it's become pressurized), take the bottle and put it in the fridge. This will slow down the fermentation and keep things from exploding. It never takes more than a day or so, but depending on how you vary the ingredients, things may take a little longer.