I use Sigil. You can find it here. It is free. You can download Sigil and soon be generating ePub files to publish. I spent about a day learning how to use it.
One of the good things about Sigil is that it'll read pretty much any unencrypted ePub file. If you see someone else's ebook and wonder, "How'd s/he do that?" You can use Sigil to reverse-engineer it. I'll talk more about that later.
For now, let's just stick with the basics. After you download Sigil and read its online documentation--or skim it at least. You should run it on your machine. It'll look something like this:
Now, get the HTML files you converted in the last step of this process. Click on that little blue cross on the toolbar and that will enable you to add your HTML files to the project. Put them in the folder marked Text.
You may have some images that your ebook uses. You have to copy them into the Images folder. Use the right-click pop up menu. If you have a lot of images, this can get involved.
If you have images like this, then you have to go into the HTML markup and change the locations to point inside the epub file. (If you're confused, don't try to grok this in one go. Just take this as a placeholder for something you have to learn to do, or hire out to a teenager.)
(You can find the bullet-point outline of How To Publish An Ebook here.)