How do you rebuild a carb for a 99 harley sportster?

If the carb is he original Keihen CV carb, rebuilding it is not for the amatuer mechanic. This is a very complicated carburetor as compared to an S&S carb. There are a few parts in the carb that will fit in either way but only one way is correct. But if you wish to give it a shot, simple take the carb apart from top to bottom. Carefully observe how EVERYTHING is positioned in the carb. Remove all jets and pay particular attention to the needle jet as it goes in with the beveled mouth to the top of the carb. Do not install this upside down. Remove ALL rubber components including the diaphragm and soak the carb in carb cleaner overnight. Rinse and reassemble the next day. Make sure you do not pinch the vacuum diaphragm. I'd replace the accelerator pump diaphragm in a carb this old.

Good Luck
steve