For Integrating your Pre-Existing WordPress site with LearnDash just follow the steps that are mentioned below.
Download LearnDash with its license
Now when it’s downloaded then simply upload the file with Upload plugin option
Want to Know How to Upload Plugin?
Now, Install and activate the plugin and create courses, lessons, etc.
Want to know How to Add courses to the menu?
Before that make a page with the courses name and list courses on that page.
Want to know How to add courses and Lessons on a page?
Now just go to main site and your course menu is visible there.
Enjoy with LearnDash.