Add a link to your companion instance on port 8081 to the redirect URIs (ie. if local add http://localhost:8081)
Add the restream module to companion
Add the Client ID and Client Secret from your restream application
Add the redirect URL that you used in step 2
Click Save
If you are using the same computer that companion is running on, restream should pop up automatically. If you aren't, then go back into the restream module and copy the Authorization URL.
Click Allow and you should be redirected to your redirect url and get a message that says the code was recieved successfully.