The vast majority of the time, I send a structured three-way email. My first paragraph is dedicated to the person that I'm referring. I provide a basic rationale on why I think the person I'm referring them to fills a need that they have. If there are procedural steps they need to go through (like with an estate planning attorney), then I'll explain those.
The second paragraph is intended for the person that I'm referring to. I'll help to understand why I felt this person would be a good referral for them, and if appropriate, I'll map out the current situation.
Lastly, I'll include the critical contact information both parties need to contact each other.