Plain English is a skill, and it's not an easy one to learn or to practice. It's easy to misunderstand 'plain English' as the use of simpler, commonly used words — but that description on close analysis isn't really very meaningful. The words people choose vary geographically, by social group, education, and by profession. Plain doesn't mean insultingly elementary or childlike. Plain English is therefore determined by yourself as the narrator, your reader, and their comfort with technical language and jargon (perhaps counterintuitively, both can be essential to keeping things plain).
It's also a matter of limiting your words; cutting out excess and complication where it isn't required. Using words that are concise but also familiar. Complicated concepts can often be stated more eloquently with a single technical word, but only if your audience is confident of it's meaning. Learning to edit and cut away at what you've written can be the hardest part of writing, but is also one of the most beneficial processes; our writing will often follow our meandering, wandering, train of thought. Another human can be a great help as you will be prone to making the same mistakes when you review your own text as you made when you wrote it. Even with the greatest objectivity you can't account for your own natural, human, errors so easily.
Not all audiences are easily predicted or understood; if you write about a wide range of topics you're likely to attract an even wider diaspora of individuals. You need to balance the language that might be comfortable to a broad spectrum of people with the language that you might use in conversation yourself. There is no perfect lexicon of universally understood English, so don't be afraid to trust your own voice. You most likely speak english effortlessly every day, so you're practiced at avoiding confusion. If something feels wrong to you then it probably is.
Thoughtfulness and consideration therefore are your best tools. Read your own work. Think about the jargon you might have used and whether or not it really does add clarity. Consider whether the language flows naturally. Cut out any needless explanation or rambling text. It will get easier.
I'm still figuring it out.