tolkiens legendarium - Is there any procedure to be followed when an half-elf gives up immortality?

I went through the following questions, Why can Arwen decide her mortality?. It explains who can give up immortality. I would like to know if Tolkien had ever mentioned a procedure/ritual that has to be performed to give up immortality or it's just a decision in mind to give it up?

Secondly can this decision be reversed?

Answer

There's no mention of any kind of magic procedure, spell, incantation, etc. needed. Half-Elves are simply born with the option to make a one-time-only choice between immortality or mortal life with an unknown afterlife.