
How to ? First buy a commemorative, decorative stamp, to affix on the envelope.
Then, find some good writing paper, the acid free kind that can last the years. Sometimes this is sold by the sheet.
A good pen - flowing gel ink for instance - and your best handwriting are key.
Of course writing FROM YOUR HEART is the best and only thing to do.
Written, walk that letter to the mailbox and put a kiss on it before you mail it.
Vibes... the best!