End-user

Go to Special:CreatePage as a user who has the createpage user right. Additionally, if the $wgCreatePageCoverRedLinks option is set to true in the wiki's LocalSettings.php file, users can choose on their preferences (Special:Preferences) to use the CreateAPage editor for all content namespace page creations.

Wiki administrator

You may need to customize the message MediaWiki:Createpage-template-infobox-format, especially if your wiki's content language is not English and your infobox templates are not prefixed with the prefix "Infobox". This message contains a regular expression that is used to determine what is and what isn't an infobox for the purposes of rendering an infobox via the CreateAPage interface.

Once that's done, you need to set up some createplates. Create the createplates you want (e.g. MediaWiki:Createplate-actor, MediaWiki:Createplate-character, etc. for a wiki about a television show or a movie/movie series) and ensure that these contain the necessary special markup needed; refer to the old Wikia help page for more info.

Once that's done, edit the list of available createplates at MediaWiki:Createplate-list to make the new createplates usable at Special:CreatePage for users. The syntax for that page is createplate name (e.g. actor in MediaWiki:Createplate-actor), followed by a pipe, followed by the name of the createplate as it should be displayed to end-users on Special:CreatePlate. Entries are separated by newlines. For example:

actor|Actor or actress
character|In-universe character
Blank|Other

You'll likely want to keep the blank createplate there to ensure nothing breaks and to offer users a less structured approach at creating a new page, because there are no doubt some cases where that is entirely warranted.