An outline of a talk by John Stott:-

I. Doctrinal Aspect
“Witness” q.v.

II. Practical Aspect
A testimony is NOT (necessarily) a biography, i.e., sketch of circumstances of one’s conversion, BUT a recommendation of Christ. A testimony is a testimonial—recommending Christ from personal experience.
Its purpose is to advertise or uplift Christ as:

• Savior who found and forgave us
• Keeper who is holding us fast and leading us on
• Master who is teaching us His will
• Guide who is revealing His plan
• Friend who never leaves us

Therefore—

(i) Be humble. Acts 18. It’s a question of emphasis: Not “my experience of Christ” BUT “Christ, in my experience.” It’s a subtle distinction, but real. Leave people with Christ. Chief pronoun neither “I” nor “you” but “HE.”
(ii) Be honest. Easy to exaggerate in order to impress (e.g., power, peace). Easy to use pious clichés (“I’ve often let Him down but He …”) Don’t be glib, or use jargon.
(iii) Be helpful. Remember your audience. Talk of Him in a way which will help them. Don’t divulge intimate secrets (e.g., marriage, jealous or impure thoughts). No use telling a girls’ school of Christ’s keeping power in army nor children regarding honesty in business.

The Preacher’s Notebook (ed. Mark Meynell), Lexham Press, 2018.