Some things that might be obvious but werent to me at the very begining.
Put toilet paper in the toilet before they go so they dont ‘hear’ the noise and its not so scary.
Use a good toilet seat, ones with handles, so they feel secure sitting on the toilet.
Let them read a book or listen to music etc. to take their mind off ‘the job’ so to speak.
Let them see you doing it so they know its okay to do this.
Let them ‘play’ with the toilet maybe putting some paper in and flushing it away so they get over the fear of the water and the noise of the flush.