The purpose of the assessment is to provide the starting point for a new measure that will help parents understand how well schools support their pupils to progress between reception and year 6.
The RBA is a short, interactive and practical assessment of your child’s early literacy, communication, language and mathematics skills when they begin school, using materials that most children of your child’s age will be familiar with. It is statutory for all schools from September 2021.
You do not need to do anything to prepare your child for the assessment. Your child is unlikely to even know that they are doing an assessment when they are completing the tasks.
Here is a link for more information regarding baseline - https://www.gov.uk/government/publications/reception-baseline-assessment-information-for-parents