Here's a great Thnks blog post to get you started - Hacks for Writing Thank You Notes!
When expressing gratitude to someone it’s better to let them know they met your needs than to recognize the cost to them. Eg- "Thank you for your time, it really helped me understand that thing better" is more effective than "thank you for your time I know you’re quite busy"
Be sure it's a Thnks not a "Th-ask"
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