Step 1: Study anatomy and behaviour (of people, or animals, or whatever your main characters are most likely to be). Study it well enough that you can draw the characters from any angle, in any pose, pulling any facial expression that you might need for your story (and preferably more).
Step 2: Study things like story structure and literary devices. Irony, pacing, story beats, how characterisation works, etc. You might have a great story idea, but can you turn it into a great story? Even if you're writing a gag-a-day comic rather than a story-based one, being able to write good stories is a huge benefit.
Step 3: Read Scott McCloud's Understanding Comics and Making Comics, which will provide you with the framework and vocabulary you can use to understand the needs of comics as story-telling media, and to communicate with others (so you can understand other people's articles about comics better).
Step 4: Armed with the knowledge to create engaging stories and how to show sthese stories visually visually in an effective manner, put it to practice by starting to write and draw your first comic. I recommend doing something other than the comic you really want to do. Make some short one-shots, so that you don't feel bad moving on to something else when you quickly improve to the point where they seem like crap.
If you start your main project as your first project, it's very likely that you'll be tempted to restart it every time you improve a little, and you might lock yourself in a cycle of restarting. This is not good for you (since you wouldn't be drawing as many different things as you could be), not good for the comic (which wouldn't progress), and not good for the readers.
Learn the ropes with some simpler, shorter projects. Once you feel you've got a decent grip on visual storytelling, start your comic. You will keep improving, and you will come to hate your early pages, but because they'll at least be done competently enough to be understood by the readers, it'll be much easier to avoid the temptation to restart the comic unnecessarily.
Step 5: Build up a buffer (if your comic is meant to react to current events, then this might not be viable). With some extra drawn pages serving as a buffer between where you're drawing and where you're updating, you can safely take some sick/busy days off and still maintain a consistent update schedule. The ideal buffer size depends on your drawing speed and consistency, and the update schedule (which should also be consistent). I recommend having a month's worth of updates as a buffer (so if you update once a week, that's 4 pages, if you update three times a week, that's 12). I personally like having 3-4 months' worth for my long-form narrative comics, and a months' worth for anything else.
Step 6: Find a way to get your comic online. These are your major options, though not the only ones:
- Create your own site. You'll generally need a paid webhost, because unless you want to code the webpage for every comic page yourself and waste valuable time, you'll need the ability to have some automation. For this option, you'll need to have some coding skills, or the ability to pay someone to do it all for you. You can install software such as ComicCMS or WordPress to make managing your comic easier.
- Use a free comic host. This is the most common option, and the easiest to google for. These are free services that provide you with a comic site and all the tools you need to update easily, with no coding requires. Most of these, however, put ads on your comic. For your first big project, which you will most likely not be monetising, this shouldn't be a problem. If you want to customise these, you'll want to learn how to do some HTML/CSS coding.
(Not all sites that let you put your comic online are webcomic hosts. Some are optimised for non-webcomics, such as sites that normally host scanned manga. Don't use these. You want something with by-the-page management, and ways for your readers to engage with you, such as comments.)
Step 7: Start updating!
Step 8: Keep updating! This is where that buffer from step 5 comes in handy.
You might not get many (or any!) readers right away. It can take a while for people to discover that your comic exists. But, if you did steps 1-4, then you should have no trouble keeping the readers that do find your comic. Once you have enough pages up for people to get a good idea of what the comic is like (probably 30+), you should do some advertising to get more people to discover your comic.
I'm not sure whether you wanted all of that, or just a detailed explanation of how to actually sit down and write/draw your comic (step 4). If you've done steps 1-3, then step 4 should be self-explanatory. However, if you still need some guidance, just say so. Also, take a look at other threads here in CC, we get a new "how do you write/draw your comic?" thread just about weekly.