From the My TV tab in the Sling TV app, scroll down to the My Channels ribbon.
Scroll over to the far right to the end of the ribbon and select Edit.
All of the available local and Sling TV channels will appear. From here you can add any channel you want by selecting it. A green checkmark will indicate, in the upper right-hand corner, that the channel has been successfully added.
Once you have chosen all of your favorite channels, hit the Back button on the remote. Now your channels will appear in the My Channels ribbon on your My TV screen.