Step 1: Log in via Plesk
Creating a staging environment can be done in the Plesk control panel. You can read how to get into Plesk here.
Step 2: Go to your WordPress installation and clone to a (sub)domain
In Plesk, click on 'WordPress' in the left menu and, under the WordPress installation you want to create a staging environment from, choose 'Clone'.
Then a screen opens where you can choose a location for your staging environment on a specific (sub)domain. Here you can create a new subdomain or choose an existing domain or subdomain. The entire WordPress website will then be copied to the specified location.
Step 3: Transfer your staging site back to your main site
Once you have finished developing the WordPress site and it has been successfully tested, then it is time to transfer the result of your work back to the main domain.
In the WordPress toolkit, choose 'Copy Data'. There you will see a menu where you have several options. As a rule, you can leave the default settings in place if there have been no changes to your main website during the time you have been working on the staging website.
If, in the meantime, there have been changes on your main website, then you can uncheck the 'Except: wp_postmeta, wp_posts, wp_usermeta, wp_users' box under Database tables.
When this option is checked, all articles and users from the main website will remain intact.