Subtitle files must be in the VTT format and have valid formatting. A number of tools are available to help produce subtitles, such as:
- VTT Creator – which can be used in the browser, so there is no need to install any software
- Gaupol subtitle editor – this is a program which needs to be downloaded and installed
You can check the formatting of your VTT file is valid by using an online validator tool, such as the one at: https://quuz.org/webvtt/ (please note that this validator will show formatting errors in YouTube generated VTT files, though they will still display in the Video Portal).