Pretty easy job, just a bit messy though.
Firstly I would double if not tripple check the oil leak is actually coming from the sump seal as mentioned.
My experience with all these engines is that 98% of the time, its actually coming from either the cam seals or oil pump gasket or seal & running down to the sump.
If it is the sump though, heres a basic guide.
- drain oil 1st, I would let it sit for a day or so draining, just makes for a cleaner removal.
- remove the sump pan (approx 15 x 10mm headed bolts + 2 10mm nuts, pry off carefully, as old silicone will be holding it pretty well.
Once removed, this it the hardest part, ensure you totally remove all old silicone from both the sump pan & the block assembly. (can be a little tricky when removing it from the block unless you have a car hoist - as most people out there, you will have the car jacked up & on stands & lying on ya back with limited movement).
Clean both surfaces properly, use something like thinners, mineral turps, gunwash etc.
Purchase a good quality oil resistant tube of automotive silicone (approx $15 from any autoshop), run a nice clean bead only around the sump pan (go a little heavyer on silicone around the edges too) and re-install sump back to block using all bolts & nuts.
Allow at least 24hr to cure, then refill engine with engine oil & hopefully that fixes the leak.
Hope that helps in some way.