This works best with a styrofoam cup with a lid.
All you need is:
-One cup with lid that is in use
Step 1: Make It Fast
Wait until your prankee leaves their drink unattended.
Tear a small hole in your lemon packet
Take off the lid and straw and slide the straw into the lemon packet then replace it, submurging the packet into their beverage.
Step 2: Wait For It
It's happened to me before and it was quite the surprise when you're expecting to taste sweet soda.