The National Foundation for Educational Research (NfER) says that from the start of the next academic year a sample of schools will help it gather feedback from teachers and children about the testing process.
The organisation, awarded a £10m contract in April by the Department for Education to develop the tests, also confirmed that questions will have a strong focus on assessing children's literacy and maths skills.
The baseline tests were announced last year by the government as a way of assessing children's progress throughout primary school.
But the plans have come in for criticism from academics, education experts and early years leaders, who say testing at such a young age is unreliable.
Earlier this month an expert panel established by the British Educational Research Association concluded that the tests are "flawed, unjustified and wholly unfit for purpose".
The baseline test will involve a teacher or teaching assistant asking questions during a one-to-one session using picture prompts, according to the NfER.
The test is expected to take around 20 minutes and be completed within one session or a series of short sessions, "if the teacher feels that is better for the child", says an NfER update on the trials.
Questions also get steadily more difficult as it progresses and children won't be asked harder questions in areas where they haven't completed easier ones.
Teachers or teaching assistants will record responses electronically. All children will also be assessed in the same way and the process will have a strong focus on avoiding bias, says the NfER.
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"The trialling process is not just designed to inform the design, robustness and validity of the proposed assessment; it is also a critical opportunity to gain feedback on how children and teachers experience the assessment," states the NfER update.
The trials of the baseline test will take place until next summer and ahead of a national pilot during the 2019/20 academic year.
The test is set to be rolled out to all schools in England in 2020/21.
The NfER update adds: "The proposals for this new assessment have been informed by international research evidence and our extensive experience of assessing reception-age children, and will be subject to robust trialling of assessment questions and materials with teachers and children."