Ask HN: How to scope a bespoke CRM project?

UK SME Company I work at is looking to commission a custom CRM that meets are needs.

It's all in my head! How do I get it out?

Is there any guidance / manual on how to describe to programmers what we want?

How does one put on paper specifications?

Want to KISS but even so somewhat complicated given sector.

(separate note - Do I insist also on html/js/php which I'm competent in order to keep tabs? Or best to leave to what they are more competent in.)