Easy. First, write down your engine make and model and call a Masteraft dealer and ask for the impeller part number for your engine. You can also go to somewhere like a West Marine store and the parts department can help research that info too. Keep this info because you will need it the next time you change the impeller. Next, Google 2001 Mastercraft manual and download the manual which show you the location of the water pump which houses the impeller. From there is merely a few bolts to remove the impeller cover plate from the water pump, then pull out the old impeller. Push the new one in and replace the cover. There is more than likely a gasket behind the cover...use white lithium grease to hold the gasket in place when replacing the cover and it also helps improve the water tight seal.
Also, google & youtube "how to change a boat impeller" or something similar to find some how-to videos. Good luck!