To use your own error pages, be sure custom error documents is selected for the domain under Setup for the domain in Plesk.
Create the new documents and upload either to httpdocs or to a subfolder of it such as httpdocs/error_pages.
In Plesk, click on Web Directories.
Click on the Error documents tab.
Click on the error you want to change the document for.
In the type drop down list, select URL.
In the Location box, enter the name of the error document with a / in front if it is under httpdocs (i.e. /someerror.html) or enter /subfoldername/someerror.html if under a subfolder. (Change subfoldername and someerror.html to the names you created)
Click on OK to save your changes.