A service manual is best, but here are the basics: Drop gearcase. Remove nuts and screw(if equipped) from water pump cover. Remove slinger and pump cover. Inspect SS liner for scratches. If scratched, replace it. Remove impeller and save drive key. Remove SS wear plate and two gaskets.
If in doubt about quality of driveshaft seal, replace water pump base and O ring. To do that clean driveshaft with sandpaper and solvent. Pry out waterpump base with large screwdrivers, Do not damage casting. Lubricate driveshaft and install new pump base with correct new oring and gasket. Grease base gasket and Oring.
Install new gasket to match base, then new wearplate, then new gasket to match pump cover. Now install impeller over drive key. It only fits one way.
Lube impeller with oil or soap. Place pump cover over impeller and turn driveshaft CW while pushing down on cover. After it seats, snug bolts and screw. If you had the base out, it may not be seated properly, so make sure it is inplace.
Lube driveshaft and shiftshaft splines, and install gearcase in neutraha