Usually nappy rashes occur due to constant contact of skin with urine or feces.
How ever fungal/ bacterial infections/ allergic rashes do occur in that area.
For any type of rash, the important thing to be followed is to keep nappies away and keep the area open for a couple of weeks at least till it heals. Else it will not heal and will recur.
After baby passes stools, do not wipe the area hard. Just give a gentle mop with a wet cloth.
You may use barrier creams like Siloderm that will protect the affected area from urine and stools.
If its fungal and itchy- then canesten
for a week will help.
Hope my answer helps you.