I've written a few on that topic. Here's one - you don't need all these elements, but the more you have, the stronger the pitch...

https://medium.com/career-paths/writing-the-perfect-pitch-for-magazines-and-newspapers-cee8ea68d1f3

And another one here: https://medium.com/@susiekearley/how-to-sell-feature-ideas-to-magazines-3bb9c5bf4776