Before you start
You need an account that may create and publish pages, and you need to know which layout the page should use. The layout decides what the page looks like and which fields you fill in, so choosing it first saves rework; Choosing a layout helps.
1. Create the page where it belongs
Open Content » Pages. A page is created under another page, its parent, and the parent's address becomes the start of the new page's address. Find the parent in the list and click the "plus" icon on its row (Add subpage).
2. Name it and choose the layout
On the Content tab, give the page its title; it is what the admin lists and what the page usually shows as its heading. Choose the layout. The fields below change to match the layout you pick.
The Details tab holds the slug, the page's own part of the address. Leave it empty and it is made from the title when you first save; it does not change if you rename the page later.
3. Fill in the areas
The fields are the layout's areas. A Text area is one line, Richtext gives you a formatting toolbar, Image opens the file picker, and a Component Collection lets you add blocks in any order with "Add component" (see Working with components).
You do not have to fill in everything. A well-built layout leaves out a section whose area is empty instead of showing a gap.
4. Save and look at it
Save and activate the page. Live Preview shows it as you edit, and once it is saved, "View in Frontend" in the page's menu in the list opens it on the site.
5. Publish
A page is visible to visitors only once it is published. Switch on Published on the page's row in the list, or set its publish date. Until then its address answers "not found" to everyone who is not signed in to the admin, which is what should happen.