Consider the situation. You can't just do a sarcastic laugh at anytime. You'll have to learn to judge whether or not the timing works.
Use your tone to make the laugh sound sarcastic. You can still make the "haha" sound, but your tone of voice should be completely different in order to convey your sarcasm. Usually a very dry tone works well.
Draw out the laugh dramatically. You can't giggle cutely during a sarcastic laugh. Your laugh is slower with more breaths used to make the sounds.
Keep the pitch of the laugh in mind. You should use a lower voice to laugh in a sarcastic manner. If you can get your voice very deep, this works even better.
Perfect a look to go along with your sarcastic laugh. Your eyes should convey that you are serious and not really laughing. You can also add a slight eyebrow raise for an added effect.