Hi Umgowa. I've been doing this for years (with NF), so I'll share what has worked for me. (I'm new to fiction).
When I first started, I used one of the sample letters from Writer's Market (the book).
Basically, it's like this:
1) 1 paragraph on why I'm contacting you: i.e. I read in your guidelines that you're looking for articles on X, Y, or Z.
2) 1 paragraph on what I want to write about and specifically the "angle". For example, not "I want to write about kumquats" but "I want to write about kumquat myths because they are so misunderstand and the information on the internet is rife with conflicting information."
3) 1 paragraph on why I'm the best person to write this article for you. I've been a kumquat farmer for 60 years and my articles on fruit have appeared in X, Y, and Z mags. Or I have a degree in Kumquat economics.
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Exceptions:
Once recently I contacted a mag for guidelines, and the editor emailed me back with "we don't have any guidelines. what do you want to write about?" so it was much more informal, and basically I just sent him a hi how are you, here's my pitch. (and I write regularly for them now).
Once I contacted a mag about writing a specific article, and editor got back to me saying "nah, I don't think that applies to our readership..." But I thought he was wrong. So I spent a week gathering published stats and contacted him again with "I know you said no to this idea, but I think you may be wrong so with all due respect, let me show you some stats on why I think this is so important...." and I got 6 articles out of the deal.
So you have to be creative and think on your feet.
I hope this helps