If you are wanting to do this with her then the best option is too simply talk with her about what she wants/expects from it and what you would be willing to do, there are so many different ways that people experience this that it is near impossible to make a complete guide for what you can do and definitely impossible to make one that fits everybody.
Ask her about what she likes to do, how she likes to talk/be spoken too, how she wants to be treated etc.
she might just be a person that likes the idea of a "daddy" partner, no diapers, no ageplay, just a daddy to take control of her and make the decisions (more a dom/sub ddlg relationship).
She might just be a bratty sub who wants a daddy to reprimand and punish her and make her do things.
She might be into a little ageplay and wants to have children's toys/play kids games/be treated like a child, what she wants here could vary depending on the age of play and her own experience and ideals. This may or may not involve actual regression where she enters a mind set of a child, this "littlespace" can be quite a strong compulsion or she might just enjoy letting loose and playing dong things that remind her of/make her feel like being little.
She may or may not want to wear diapers or training pants, and if she does want that part she may or may not be willing to use them. she might be more into "older ddlg" play where there are no diapers but may be curious about potty "accidents".
She may be a little more AB oriented and want to lose full control of her choices only being allowed age appropriate toys for a baby, kept in diapers, fed, changed and dressed, given bottle/paci's etc.
The only way you find out what you can and can't do with her is to talk it over with her and then try stuff out, over time things may evolve and you find either she or you wants to try other things.
I know this is not a lot of help but without knowing her or what she likes/wants it's really hard to say what you can do, if you get some information from her about what ddlg means to her we could be give more idea's about what you could try.