A lot of the liturgical book entitled “Rite of Christian Initiation of Adults” is available at
catholic-ew.org.uk/liturgy/Resources/Rites/RiteRitual.html#RCIA . This has the introductions for the book used in England and Wales. The USA edition has different paragraph numbers.
On page 41 of the PDF file there is chapter 5: “RECEPTION OF BAPTISED CHRISTIANS INTO THE FULL COMMUNION OF THE CATHOLIC CHURCH”.
This section includes:
“391 The baptised Christian is to receive both doctrinal and spiritual preparation, adapted to individual pastoral requirements, for reception into the full communion of the Catholic Church. The candidate should learn to deepen an inner adherence to the Church, where he or she will find the fulness of his or her baptism. During the period of preparation the candidate may share in worship in conformity witli the provisions of the Ecumenical Directory.
Anything that would equate candidates for reception with those who are catechumens is to be absolutely avoided.”
While the Catholic Church is likely to accept a Protestant’s baptism and Eucharist, it is unlikely to accept their confirmation. I think this is due to its understanding of authority of Protestant bishops. So there is also likely to be preparation for one of the three sacraments of initiation, confirmation.