Welcome Aboard !

It involves removal of the front and inner engine covers, a repair accurately described in every shop manual. So I'll pass on a full description.

I'll only add...
1. Remember to disconnect the battery first
2. You'll need an alternator puller tool
3. Removal of the inner cover most always involves multiple repairs and updates. It is absolutely NOT worth the effort to do a single repair. These tasks include...
• Cam shaft oil seal replacement
• Ignition can ("bean can") o-ring replacement
• Cam chain replacement
• Cam chain guide replacement (the moving one)
• Cam chain guide tension spring update
• Installation of the solid rectifier board spacers
• Inspection of the white, 3-position ignition can connector
• Checking/ replacing the 2 alternator brushes
• Replacement of the 3-piece inner cover gasket

Once you have the puller, this is straightforward work most any owner can complete with the tool kit. The oil seal installation task will be hugely aided by applying a product called simply "P80". https://www.ipcol.com/assembly-lubricants/p80-emulsion

Hope this helps.

it depends on which oil seal is leaking. There are only two seals: a large lip seal on the crankshaft and the beancan o-ring for the the cam shaft. The beancan is a straitup deal where you mark and pull the beacan, slap on a fresh o-ring and replace it. It is often very tight so use lube like Wobbly says.

The Crankshaft seal is behind the alternator rotor so remove that, pry the seal out from the outside of the timing case, push in the new seal and button it up. Observe the old seal before you remove it so you get the correct depth, I believe it goes in flush. Then put the alternator rotor back on with all tapers clean, not so much as a fingerprint.

if the beancan was disturbed hook up a timing light, connect the battery, start the bike and confirm the timing.

If the alternator was disturbed hook up the battery, start the bike and monitor voltage at the battery to confirm charging.

In both cases unhook battery before replacing front cover.

Edit: it is quite possible neither seal is leaking. If you are getting oil and crud build up on the chin of the engine below the front cover disconnect battery, pull front cover (ain't that oil cooler fun? I modified mine so it needn't be removed) and look for trickle marks of oil under the cover. If not found then the bolts and seam to the timing chain chest/block are suspect or possibly the joint at the front of the oil pan.

It would help to mention how many miles are on the bike.

two of the bolts that hold the timing chest to the block have a gasket on them on the 'wet' side of the timing chest. That gasket can leak or be missing resulting in weeping around the bolts. Most guys just patch this with some silicone on the outside until it is timing chain time and the chest is pulled at which point all the internal gaskets get replaced.

Hello,
Thanks for the help for my 1983 R100RT. I had risen on a few km of very fresh asphalt and that caused most of the mess on the oil pan. I guess the tar stuck because of the hot pan. After cleaning everything and going for a ride, I found a leak on the inner cover gasket that i patched with col weld. It seems to be now under control.
Thanks,
Ted Smith,
Baie-Comeau, Quebec.
p.s. I am looking for an original seat in excellent condition to replaced the Corbin that I had installed long ago.

That's good news, but continue to watch that area. In my experience, externally applied sealants never seem to last very long.

Also check the 4 push rod tube seals. They are famous for covering the oil pan with oil.