Step 1 — Install Fake Webcam & OBS
Download and install Fake Webcam and load a video. Also install OBS Studio (free) which is the standard tool for streaming to Twitch.
Step 2 — Add Fake Webcam as Source in OBS
In OBS, click the + button under Sources and select Video Capture Device. Name it and click OK.
Step 3 — Select Fake Webcam
In the properties dialog, set the Device dropdown to Fake Webcam Source. Click OK. Your video feed will appear in the OBS preview.
Step 4 — Connect OBS to Twitch
In OBS go to Settings → Stream, select Twitch as the service, and enter your stream key from your Twitch dashboard.
Step 5 — Go Live
Click Start Streaming in OBS. Your Fake Webcam feed will broadcast live to Twitch.