After you are in the edit view of a page, create a link by first highlighting the text you wish to link, then clicking the link icon in the Body field to bring up the link dialog pop-up.
Link icon:
If the page you are linking to is internal to your site, start typing part of the title of the page you would like to link to into Search for content in the link dialog. The page should appear in a drop down if Drupal is able to locate it. Select the full page title from the drop down and notice that a new link appears in Link URL with the form of node/123. Note that the full easy-to-read link will be displayed to the end user when the site is displayed. Using the node reference instead of a full link makes it easier to change URLs without having to track down dead links.
If the page you are linking to is external to your site, simply enter the full URL into the Link URL field on the Linkit dialog (eg. https://www.externalsite.com).