Well, you need the gearbox and clutch off of course. Then you need to take off the black plate, which is just 1 or 2 little bolts. It is best to have the sump off, but you don't need to. The retainer has a few little bolts, 10mm socket. Easy. Then to get it off once everything is undone you may need to lever it off.
To put it back on I don't use a gasket. Just silicon.
Put some grease on the inside lip of the oil seal. The seal should be pressed into the housing dry.