A simplified explanation but this job is quite straightforward if you have some mechanical experience.
Before you start download the Workshop Manual from the Manual Section at the top of the forum page. Then read the section on lower crankcase removal (= Sump Removal).
You must remove the "T shaped cross" (= Prongeron) to get at the sump.
You must also remove a section of the exhaust system.
Once jacked up, put axle stands under both jacking points under the "A" posts. Remove wheels and support the suspension with 2 more axle stands, but do not raise the suspension: just support it. Once these are in place DO NOT MOVE THEM until the job is finished.
Undo prongeron bolts one by one then remove prongeron. This turned out to be quite easy as all my bolts were reasonably free and not rusted.
Remove the front part of the exhaust after the cat box. If rusted use heat on the studs and nuts. You may need to buy new ones which are readily available.
You can now undo all the sump bolts and remove the sump. Do your repair. Clean all old sealant off sump/engine block, then replace sump with new sealant. There is no gasket (French: jointe)
Whilst the sump and prongeron are removed, change the oil filter: access is very easy!
Replace exhaust then replace prongeron after giving it a good clean and perhaps a respray!