NOTE: You can only cross-list a class at the START of a course. If you try to cross list after the course has started, you will lose all submitted work by students as well as anything you edited in the course.
Click on the class you wish to put into or cross list into another class. Basically, you are going to dump this course into the course you are using as your "master" one.
Click on Settings in that canvas course (not the one you are using as the master).
(Make sure this is the one you want to "dump" into the "main" course.) I also suggest looking at the title/name of the course you want to use as your main course before you go to the course you are "dumping" into it.
In Settings, click on the sections tab along the top of the website. Then click on the course section.
Click on Cross-List this Section in the menu on the right. This should open up a pop-up window.
In this new window, enter in the name of the course in the search window. You can also use the course's ID to find it. I just start typing the name that the district already assigned my course to find it.
Once it pops up under selected courses, select the course you want to "dump" this class into. For example, I was dumping my Period 7 BIO HON into my Period 5 BIO HON.
Click on "Cross-List This Section" in the blue box on the bottom. Now those classes are both in the same course.
You can repeat this again if you want to cross-list more than 2 classes. Just be sure to start in the class you want to MOVE into the "Master" class.