DD isn’t about regressing your partner to be a baby 24/7 so your day-to-day interactions and relationship shouldn’t change a great deal. The DD rules you pick will dictate the level of baby talk enforced in your relationship.
You should however make talking about their DD normal and include it in any pet talk you might already have. Interestingly referring to your partner as “baby” is one of the most common pet names people use so this if very safe to use publicly.
Talking to them as if they’re a baby when checking and changing their diapers is a good idea and will help reinforce their position as a diaper wearer who needs your love and care. You can also set a rule they can’t use adult words during this time.
Some couples find having a baby day to be fun and freeing, especially if your partner has adult baby tendencies. This is covered in detail in the rules section and will see your partner play the part of an infant for a day, including no adult words. It’s hard work so we recommend doing it max once per week.