Create things. Tell people.

I'm making broad assumptions about what you're good and bad at. Read the following through that filter and throw out anything that you are confident doesn't apply.

1. The best first step for this is to begin writing regularly on the topic in public and get comfortable being more self-promotional than most developers are. I suggest blogging since it's approachable and people regularly read blogs. Think of your blog as an MVP. It will help you determine if there is an audience who are interested in this type of content and where they hang out online. Knowing these two things is critical in successfully promoting and selling an eBook.

2. Most people that I know who are successful at this believe that you should begin doing email capture straight away. Basically, you want to give people a mechanism by which to receive regular updates from you. If blogging is the simplest MVP for writing, the simplest MVP for email capture is setting up a list that gets emailed every time you create a new blog post. I'm an engineer, I get it, that's what RSS is for. It works. Some subset of people who are keenly interested in a niche subject are happy to get emails when something about that subject is published by an authority that they trust.

On this one, you don't want to stop with the MVP. You need to step it up and build a list of people who are comfortable opening email from you regularly. A good step in the right direction is offering access to exclusive content to members of your list. You're looking for the people who would trade hard earned money for your book. So, create something that the same category of people would be willing to give up an email address for. You're pre-qualifying leads.

3. Start writing your book… while maintaining a blog, a list, and occasionally offering exclusive content to that list. To juggle these things you're going to have to learn how to create relatively thin content that's still compelling. Stuff that doesn't take forever to create but is still compelling and interesting enough to keep people around. Also, note that most of the people on your list understand that you're an individual so it's reasonable to say "I'm working hard on my book for the next two weeks so the next blog post is three weeks out."

This isn't as bad as it sounds once your comfortable writing regularly.

4. Hopefully, you've built a following of a few dozen to a few hundred people at this point. Some large percentage of whom feel like they know and trust enough to pay a bit to support your work.

The MVP here is to promote your book and ask people for their money. But like all of the above it goes a lot deeper. Guest blogging, getting influencers who have similar niches to promote, paying for promotion if it makes sense given your costs and what kind of conversion rate you can drive. This is a rabbit hole.

edit: You asked a few other questions.
Publisher: As other's have noted, you probably won't be able to land a real publisher.
Cover design: You can pay someone to design a cover for you if you want, you can also do it yourself. There is an art to cover design and I'm confident that it makes a difference but I have no data on the subject.
Other costs: Gumroad is a good service for selling this type of thing, they will take a small cut. Cover design can run you a few hundred. A good professional editor will probably run you a few grand. The software that you use to do layout will probably be somewhere between free and $200. Assuming your self funded and it's your first book, you can mitigate these costs by doing it all yourself (except for a payment processor).

Create things. Tell people.