I had a heck of a time figuring this out, but once I did it's so easy.
When you create a post use the "read more" tag in the visual editor (where you write your text) on the Edit/New post page. It's the icon to the right of the one for making and breaking links. When you click on it, it says "insert more tag". It looks like too blocks with a space between them.
Write your first lines or paragraphs and then click on this icon where you want the "read more" link to appear. It will insert a cut-off line that says "read more" at the end of the line in the visual editor (but this line won't show up on the post, only "read more" will.) This will make the "read more" link appear on the post at the end of that section. Then, whatever you write after that will appear on another page.
You can also go to the WordPress support pages and type in "read more" or "read more tag" in the search box to read the WordPress instructions for how to do this. There are a few tricks. For example, it doesn't work to type in all your text and then try to use the "read more" tag in the middle of the text. This can mess up the formatting. You need to insert it where you want it before you continue typing.