What you need to know
A testimonial helps when it explains the original concern and experience. A case study can show process and context so potential customers assess whether work is relevant.
How to apply it
Get permission, identify the service, preserve meaning, and explain outcomes with context. Link proof to the related service page.
Common mistake to avoid
Do not invent quotes, change their meaning, or publish private details without consent. A little credible proof beats vague praise.