This doesn't answer "how to", but is a caution.
An elderly man in my genealogy SS class shared this story about himself:
His parents always said they were LDS, but he doesn't remember going to church much. Missionaries came to their home when he was young, taught them the discussions, got the family to church a time or two, then baptised them. After a little while, they stopped going to church again.
A couple of decades passed and he decided to go back to church. He walked into the nearest LDS chapel & introduced himself. Several weeks later, they told him that they could locate no record that he had been baptised and invited him to take the lessons and be baptised again. Very upset, he walked out and did not return.
Some years later, he happened to run into the person who baptised him. He asked this man to write a letter indicating that he had done this ordinance. The letter stated that he had been baptised into the RLDS church. He was a recent LDS convert when I met him.
