It's not easy but basically a nickname for the movie and a type are passed into the channel app as two strings. Instead of giving your movies long serial numbers just call them by their name in all caps with no spaces like this:
contentID = "SOMEOLDMOVIE1934"
mediaType = "movie"
contentID = "ANOTHERMOVIE1936"
mediaType = "movie"
etc.
The channel form only allows so many movies in this list even though Roku says it needs way more... I only submit maybe 20-30 or whatever fits in the form's box. They will use this list to test your channel, but they also have a deep link testing channel you can install to test prior to submitting anything to Roku. I suggest that you fully get it working before sending Roku in circles with long delays that will cause you to pull your hair out.
The trick to deep linking is that in your main.brs script you need to check for the contentID and mediaType variables then store these in some global variables to be examined by your grid script. Normally when my channel is being browsed the focus method updates the title and description of the selected movie/item and displays this on the screen. If deep linking is active then you can display the grid as usual or not, but at some point you need to fire the onclick method as if the user clicked on an item, but use the deep linked contentID. You'll need to scan your entire content list for a match of the contentID and if found, "click" on that movie. It's still more nasty than this but this should get you started.
