Typically:
Remove the front sprocket cover
Remove the clutch lifter & clutch push rod.
Remove the front sprocket (likely to be on VERY tight, it's best done with a rattle gun / impact driver [used correctly])
Remove the retaining plate that retains the output shaft oil seal
Decide which seal is actually knackered and remove it. Often this is just by levering the seal out, sometimes it's easier to drive a couple of small self-tapping screws into the seal and pull it out with those.
Drive the new seal in ABSOLUTELY SQUARE.
Reassemble.
Have a beer.
arrrf.