Writing a Catholic confirmation letter involves affirming the person being confirmed, sharing your own faith and personalizing the letter. Anyone can write a letter to affirm or celebrate with a young person going through confirmation, but a letter is generally expected from the person's confirmation sponsor.
In many Catholic churches, a confirmation applicant chooses a sponsor. This person is usually a close friend or relative, but not a parent. The sponsor must be a confirmed Catholic that regularly participates in the Eucharist. Various churches and parishes have other rules that govern the sponsor relation. Among other things, the sponsor writes a letter to the person being confirmed. The letter acknowledges the sponsor relationship and responsibilities, but it also celebrates the milestone in the young person's life and gives advice about the person's spiritual walk.