installing wordpress through a test dev site on the same domain as the live site

I have my main site that runs wordpress in a folder on the root and my .htaccess is configured to pick the index.php from this folder. Now I wanted to create a dev site that will be used for testing in the same domain under a different subfolder and one that can be accessed through testing.mydomain.comor through a url such as https://www.mydomain.com/testdev. How do I do this?