It sounds funny but spanking an adult baby can occur in multiple ways.
Just to name a few. I am going to start with the fun pats to a babies buns. Yes, soft lovable mommy/baby pats to buns. Remember fun can include humiliation, add a little scolding and cute lovable kisses. Every adult baby could use some love from mommy. Fun pats can even make an adult baby go right to sleep. ( Even pats on the back of diapered buns are fun.
Punishment spankings are not meant to be fun but to get a point across. Remember adult babies have feelings and will need love afterwards, even if it is in role play mode. Environment, timing, reasoning, and understanding is needed even in the punishment scolding mode. If their is anger that is not contained by the Mommy/Daddy. The experience will be more threatening and mentally disturbing then fetish play. Establish rules for both adults before playing. Everyone needs to know limits and lines that can't be crossed. Submission during a punishment spanking must be wanted. Force spanking without consent can leave scares mentally. Safety leads to fun punishment time.
Fetish Spankings are about the positions. Yes, there are many ways to spank a pair of buns. My favorite is mommy in a chair and the little baby boy kneeling between my legs with his arms around my neck. The quick fast pats on his buns will send shocks. Having him nude adds to the fun. Remember every position and angle adds to the fun. The diaper changing position. The standing position while holding one arm, or spanking with an ear in hand are all fun to do as fetish spankings. The addition of diapers only adds to the fun.