In person

Remember, most people say that telling their partners was much easier than they thought it would be.

Do it straight away. If you put it off, you might never tell them.
Plan what you are going to say. (See the “videos below” for ideas)
Don’t feel you have to provide a lot of information.Take along a Fact Sheet or give them our website or phone numbers to contact.
If you are phoning them, ask if you have called at a good time before giving them the news. Avoid phrases like “You’ve given me Chlamydia” as it may make your partner angry and defensive.

This video shows how to tell your parther about your STI on the phone, "So they can know" is a US website for STI notification