A generic, vague testimonial does very little work in a sales letter; a specific, detailed one can be as persuasive as an entire paragraph of your own claims.
Ask for Specific Results, Not General Praise
"This changed my life" does far less than "I cut my onboarding time from three weeks to four days."
Include Enough Detail to Feel Real
A full name, and where appropriate a photo or company, makes a testimonial feel like a genuine account rather than an invented quote.
Place Testimonials Near the Claims They Support
A testimonial positioned right after a related claim in the letter reinforces that specific point far more effectively than a testimonials section dumped at the very end.
Next step: Use the Sales Letter Writer to draft a request template for gathering more specific, usable customer testimonials.