Many people wanting to write for an online magazine will possibly find it difficult to even answer the most fundamental question: What to write about? Indeed, coming up with a response to this question has proved unsurmountable in several cases I know of, discouraging the person from writing the piece.
Below, I will share how I manage to reply that question for myself, for each article I've written so far (mainly on Smashing Magazine and here, on my own blog), and for all the ones I hope to write in the future.
Deciding what to write about permalink
In first instance, there are no topics out of bounds. It could be something very simple, like a trick you discovered today by accident and that many other people could benefit from. It could also be something related to your field of expertise. Do you want to talk about the latest trends in Artificial Intelligence? Go for it! Have you installed a new Content Management System and want to describe how smooth the experience was? Sure, why not! Have you prolonged your users' browsing time on your site and want to share how you achieved it? Absolutely! Do you want to explain how you created that still-life using only HTML and CSS? Please do it!
As a starter to decide the topic of your article, you can simply find out what you are good at and focus on that. You may think that you are not entitled to write since you're not the best concerning your area of work. And you know what? You are right. But only in part: Yes, you are not the best, since there is always somebody out there with more knowledge than any of us about any topic (allegedly not even Charlie Chaplin could win a competition on imitating Chaplin).
But that doesn't mean you can't write about your topic! You don't need to be the best... You only need to be good enough, and to have the conviction to do it... (eventually you will feel very confident, but that comes with time and practice). And if you don't feel confident enough, remember that even those people we consider experts sometimes do not feel confident enough themselves!
In my case, I have a backlog of ideas which I keep revisiting and updating, and which acts as the source for my articles. This is how it works: I just start with an idea. I write my initial idea down on a simple .txt file, and let it grow over the days and weeks, adding new ideas as they come to my head, upgrading previous ones, and removing those ones that make no sense anymore. After some time, as the ideas accumulate and take a certain shape, the foundation for the article will be established. The rest is to write it.
As Stephen King explained in his book "On writing", he doesn't plan a story in advance, but lets the own characters within the story develop it. I feel it is the same for me: Having established a good foundation through the backlog of ideas, I find it very manageable to write the article, since all uncertainties of the "what" are dealt with (and also the how: just arranging the sections into a proper order, adding content, and giving it coherence).
Once the topic of the article is chosen, we gotta write it! That's the topic for my next blog post.