Before you assign a course, it's often wise to preview the course content itself. The way to do this is to assign the course to yourself. This is the same as assigning the course to an individual, but let's step through the process from beginning to end.
First, go to the Author tab and search for the course you wish to preview. Once you find it, click the name of the course to be brought to the main course page. From there, click Learners.
The Learners page will list all of the Learners already assigned to the course. To assign it to yourself, click +New Learner and then type in your own name into the new box. Finally, click on the account that comes up to assign the course to yourself.
You've now assigned the course to yourself. From here, click on the Application Switcher in the upper right, and go to My Learning. From there, you should see the course on your dashboard. If not, just search for it, and click the course. That will drop you straight into the course, and you can preview it before you assign it to people!