Whenever you find yourself on the side of the majority, it's time to pause and reflect. - Mark Twain More Sarcastic Quotes
This will be the hardest portion of your studies, because it requires that you develop your own thoughts. As with any new venture, there will be mistakes; as mentioned before, if you are serious about learning, then just learn from it and move on.
After every attempt at being sarcastic ask questions, "Was that sarcastic?", "Did you think that my comment was clever?". Learn from your mistakes, and do not be afraid to ask for help to further develop the comments that you find are not sarcastic.
This exercise should be repeated until you can at least make a successful sarcastic comment fifty percent (half) of the time. But be sure to not become complacent, repeat your exercises once in while. The saying "Practice makes perfect" is something you should keep in mind. However, make sure that you do not overexert yourself in trying to always be perfect with your sarcasm. Just be happy with the abilities that you have developed.
Good luck!