Typically they would have you bend over (so you don’t get your clothes wet) and pour water over your forhead (3 times) saying "I baptise you in the name of the Father (pour) and of the Son (pour) and of the Holy Spirit (pour). Some churches have the capability for full immersion baptisms, but most don’t. There are additional rites and prayers associated with the whole thing, including the anointing with oil, and it can be performed as part of mass or as a separate service
As for your earlier question, I would suspect that the pastor would require what is known as a conditional baptism. Basically the same as above with the caveat added “If you are not already baptized, I baptise you …”
As CM has suggested the Church accepts baptisms from most “Christian” denominations, however it has ruled Mormon baptism as invalid, mostly due to their understanding of the nature of the Godhead/Trinity. In that JW theology on this matter seems to be closer to LDS theology than Catholic theology, it certainly raised the question, however I am unaware that the Church has ruled definatively on the validity of JW baptism.
As for RCIA, that is the normal method, and like many things there are exceptions to rules. These exceptions are at the discretion of your local priest under the guidance of his bishop, so you would need to ask them