If it is NOT a Mercury designed lower unit: Remove the prop. Remove the anode. Remove the four 1/2 inch head bolts holding the spool inside the gearcase. Pull the spool out of the gearcase. Tap down on the reverse gear bearing carrier and remove the two 1/2 circle retaining clips. Pull out the complete reverse gear/ drive dog/ prop shaft assembly.
If it is a later unit, remove the threaded retaining ring and remove the spool. Then remove the propshaft/gear/ bearings assembly.
These are very generalized instructions but you should be able to find good detailed instructions in a Clymer manual or even online manuals.
Just remember: USUALLY, not only are the drive dogs on the dog clutch bad but so are the drive dogs on the forward gear. To remove the forward gear you must remove the pinion and drive shaft--by far the most difficult part of the job.
