Step 1. When she is on her period and stuff, try to convert her away from pads and tampons. And instead suggest that she uses goodnites instead.
Step 2. After some months of this. Try to suggest to her, to wear them all of the time day and night even if it isn't that time of the month.
Step 3. After some months of this, try to convince her to throw away all of her underwears as she is no longer using them.
Step 4. After some months of this. Try to suggest her using her goodnites for what are made for.
Step 5. After some months of this. Try to convince her to switch out her goodnites to propper big abdl types of girly plastic baked diapers.
Step 6. After some months of this. Slowly buy her more and more abdl stuff, like a chaning table. Bottles, pacifiers, more and more babyish clothes. And start to bottle feed her with warm milk and baby formula.
Step 7. After about 1 & A ½ A Year of this. Tell her that this is your kink and has always been so. And that seeing her like this makes you extremly happy.