Step 5: Biological carbonation
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. The picture below shows everything in the bottle before it's been shaken.
Once you're finished, place the bottle in a warm-ish place (I set mine on top of the 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 for me, but depending on how you vary the ingredients, things make take a little longer.
Note: I am quite serious about the exploding bit. If you leave the bottle just sitting in a warm place for a few days, it will likely explode and send sugary water all over the room. So keep an eye on it!