Start with the actions the reader needs to take (be at point x at time y wearing z) and then go in to specifics and reasoning.
Every time, I then go back and move that concluding sentence to the top. That way, the email starts with what I would like to happen, and if the recipient doesn't have any objections, the remainder of the email can be skipped entirely.
(Unless it's a novel, but then the goal is to build up a story to an apotheosis)
In US schoolsg english classes teach writing as an inverted pyramid where you give details (pyramid base) and end with a conclusion (pyramid point). In contrast, technical writing and journalism teach an inverted pyramid where you start with the conclusion (point) and fill out the details (base).
Think of The Atlantic long form articles contrasted with traditional newspaper frontpage stories. The Atlantic is telling a story to build context before getting to what you wait. A newspaper article tells you who was murdered with what and where and then builds up more context.