Step 1: Sign up as developer at http://www.roku.com/developer/
Step 2: Download SDK from https://owner.roku.com/Developer/DevHome
Step 3: Read through the documentation starting with the Developer Guide.
Step 4: Continue going through the BrightScript Reference and Component Reference
Step 5: Modify the simplevideoplayer example to point to an example stream of your own to confirm that it plays
Step 6: Modify the videoplayer example to create a channel with xml feeds that allows browsing of multiple files
Step 7: Check the other examples and the Component reference for pertinent features you may want to add to your channel.
Step 8: Review the Design Guidelines and the "Before Publishing Checklist" at the end of the Developer Guide to check your channel design
Step 9: Submit your channel for publishing by Roku
Step 10: We are currently reviewing channel submissions on Thursdays. Assuming your channel meets the Design Guidelines and there are no legal issues, it may take a couple weeks after your submission for your channel to actually be published in the channel store.
This forum is a good place to ask about issues the docs didn't cover on your way to developing your channel.