Okay, so, heads-up: I don't actually know how to do this. I'm not an expert, I don't own a newspaper, etc. But, I have actually written editorials several times, and my friend who is the editor on a paper and needs to write them herself more often told me I should write a post about it. So. Step 1: picking a topic. Usually for me this means something political. To be totally honest I treat editorials a little bit like blogging. They're just slightly better organized. And from what I can tell, that's how publishers treat them, too.
But the topic can be whatever feels relevant to your readers -- at a school paper like the one I write editorials for sometimes, this often means things like safe driving and parking, vacations and finals, and local news at the school. It usually doesn't mean academics, which is probably a bad thing. We should maybe write about that more.
Step 2: Write. Literally just write whatever the hell you want. Usually I write it right in the page in InDesign, which I realize is a terrible idea. But I get to see right away if I'm filling up enough space. During this step, I attempt to hold back no opinion at all under any circumstance.
Step 3: Make it coherent. Cut off the chunks that don't actually make sense, rearrange parts that need rearranging, add paragraphs that cover background you should have covered.
Step 4: Get someone else to read it to make sure you're not being totally irresponsible. Idk if you'll need this step, but I definitely do. Step 2 generally makes it necessary.
And that's it! Basically just blog in print and don't sign it. Oh, and check your sources and don't lie.