If I have a video series that I created, how do I make that available through Roku so that others can watch it using their Roku device?
You'll need to create your own Roku channel. There are lots of ways to do that including using the Roku SDK and Direct Publisher. The method you choose depends on whether you have time and/or experience for creating a channel, and, how much you want to spend. Yes, there typically is a cost since you'll need to store your video content on a server. Having said that, Roku's Direct Publisher platform is one of the easier methods for getting up and running with a new channel.
@Baradanikto is correct , you might consider creating a Channel for your content , then share your channel with others + it will be available in the Roku Channel Store so anyone searching for new content will be able to find it.