Choose a title and filename for your page
The title is to inform humans what the page is about, can be fairly long if needed for clarity
The filename should be short, say 40 characters max. It should express what the page is about, but briefly.
Choose an index page that you think your page belongs under
Decide where in the index your link makes sense being. Maybe rearrange the index if that would improve clarity.
Edit the chosen index page to add a link to your page, like this: [[filename|title]]
After you save your index page change, the new page link will display in red, because it has not been created.
Click on the red link, which displays a page saying: “This topic does not exist yet” and “You've followed a link to a topic that doesn't exist yet. If permissions allow, you may create it by clicking on Create this page.”
Click on the Create this page pencil icon with a plus (+) sign, the top icon to the right of the editing window.
An empty page editing window will be displayed
On the first line, put the page title with 6 equal signs on each side: ======Page Title======
I suggest saving your new page at this point, so it will now exist.
Continue adding to your new page.