For the most part it isn't that hard. Both the Baby and Daddy/Mommy just have to let go and get into it. Most people that want this desire it so much they freeze up. Al you need to do is let yourself go and don't get upset if you fall out of your role often. Just go back and keep going.
Daddy/Mommy can help a lot by not referring to the baby in any way that references the baby is an adult. That means there is lots of work for them. They have to do everything from entertaining the baby, feeding, changing of course and making sure the baby is safe in the environment you are in.
The Baby has to let go of his or her big person and let Daddy/Mommy know what they need using the language and behaviour of the age they are supposed to be. A little baby will cry, a toddler might point and a little child might ask but not be exactly clear. Don't get frustrated as i said above if you use some big person words. just gloss over it and keep going. Next time you will do it differently.
Some fun and easy ways to start are with a diaper. Daddy/Mommy can get it rolling easily this way. The Baby shouldn't help unless asked during a diaper change. The most common help needed is to slip a diaper under your bum. If you are asked them raise your bum. Don't do this if you are not asked. An experienced Daddy/Mommy may know how to change you without your assistance.
While you are being changed you can suck on your thumb. Rule number one for babies, if you don't know what to do next or are a little uncomfortable, suck your thumb. That helps remind you that you are a baby. It also helps keep Daddy/Mommy in their role.
After a diaper change, Daddy/Mommy may give you a Bottle or sippy cup. Another good thing to do right after a change is to sit the baby on the floor with some colouring books and crayons. If the baby is right (left) handed they should use their left(right) hand to colour.
The biggest obstacle to staying or getting into our roles is our fear of being laughed at or thought to be silly by someone else. Remember, the Daddy/Mommy and the Baby are there for the same thing!
Part 2 - coming soon
