There are 2 different types of nappy rashes. Some are due to irritation from the urine and diaper. Other rashes that tend to occur in the nappy areas.

The first thing to do is to change diaper frequently when wet. Don't wait until there is a poo . You can Apply diaper cream - vaseline, Sudocream, shea butter, zinc oxide cream are fine.. Do not apply Powder

It must be something that creates barrier between skin and urine....powder will be formed into paste by urine and further irritates the skin.

Lastly, always allow the diaper area to be exposed to air at times when you are home...don't always cover with diaper. If it persists especially if you notice reddish colour, you need to see a Paediatric Dermatologist asap