First of all, you should figure out the year of the motor. Look on the engine block for a silver disk-shaped plug with letters and numbers. That should be the serial number. Check the maxrules web site for the model and year lookup.
Most Mercs have a spline on the lower shift shaft and a coupler on the upper shift shaft. Put the motor in neutral, remove the nuts, some Mercs have a bolt under the zinc anode (which unbolts with a thin wall socket or allen wrench), that needs to be removed as well.
Now the gearcase will slide off. Remove the slinger, water pump cover, impeller and wearplate. Replace wearplate, 2 gaskets and the impeller as a minimum. Make sure to use the gaskets that match the pump base and wearplate. Make sure to install the drive key for the impeller and lube the impeller with Dawn or something similar, and install the pump cover while turning the driveshaft CW. Tighten the bolts snug but not too tight and install the slinger.
Grease the driveshaft and shift shaft splines and install the gearcase, turning the flywheel to mate the splines.
