If they have ever worn for you, what were they wearing while they were wearing?
Was it just the diaper? A shirt and diaper? Pants/Shorts/Skirt?
Do you have any thoughts on clothing that would make them more comfortable doing it their first time? I'm not really looking for answers about you have to be comfortable with yourself to make them comfortable, I get that. Diaper wearing is a tactile thing, so I don't know if there are ways to make the sensations more pleasurable for them.