Two conversations come up here constantly. Telling someone you dated before you knew, and telling someone new before things go further. People rehearse both for weeks and describe them afterwards as shorter and less dramatic than the version in their head.
What follows is drawn from what members on this forum say worked.
Telling former partners
The purpose here is narrow: somebody gets a test they would not otherwise have taken. It is not an apology, a post mortem of the relationship, or an invitation to discuss who gave what to whom.
Short works better than long. What you tested positive for, and that they should get checked. That is the whole message. People who send a paragraph report an easier time than people who send a page.
If there is someone you cannot face contacting, ask your clinic about partner notification. Many services will contact people on your behalf without naming you. It is a standard service rather than a favour, and the person on the other end is told to come in for a test, not who sent them. Members here use it regularly, particularly for relationships that ended badly.
Telling someone new
Three things come up again and again.
Drop the speech. A rehearsed monologue is received as a confession because it is delivered as one. Information delivered as information is usually received as information. Four sentences is plenty: what you have, what it means day to day, what you do about it, and an invitation to ask questions.
Pick the timing deliberately. Not in bed, not five minutes beforehand, and not dropped into a third date as a fact about yourself. Somewhere ordinary, with time afterwards.
Know your own situation. How often you get symptoms, whether you take anything for it, what you do to reduce transmission. Being able to answer calmly reassures far more effectively than any wording, and it is the part people most often skip preparing.
Let them go away and read
The instant reaction is rarely the final one. Several people here describe a partner who reacted awkwardly in the moment and came back the next day with sensible questions and no drama, having looked things up.
Give them that space rather than trying to resolve the whole conversation on the spot.
What not to do
Do not name, describe or identify anyone else in the process. Not to a new partner, not to a former one, and not on this forum. It is the fastest way to cause real harm to someone, it is against our community guidelines, and depending on where you live it may be a criminal matter.
Nothing here is medical advice. If you want help planning a specific conversation, your clinic will have done this many times and can help you script it.