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Notepad for Windows users or TextEdit for Mac are good free options, but any text editor will do. Subtitle files utilize a specific format: subtitle number, timestamp, and text - each occupying a different line. Timestamps use an hour:minute:second:millisecond format.  For example:101:15:05:00This is a sample subtitle text. This example will place “This is a sample subtitle text” as the first subtitle at the 1 hour, 15 minute, and 5 second mark in the video. Here you can set the filetype using the the SubRip (or srt) extension (a common text format for subtitling videos). For example: ‘subtitles.srt’. The extension at the end of the name is required to set the filetype. This menu is located below the filename field. Selecting “All Files” allows the extension to be something other than plaintext. SubRip files will not function properly without this encoding set. When that is complete, press “Save”. Login to your YouTube account. Press “My Channel > Video Manager” to see a list of your video uploads. You can also access the Video Manager by clicking your account icon and going to “Creator Studio > Video Manager”. The “Edit” button and its menu is located next to the video that you wish to add subtitles to. This will take you to the subtitling interface. A popup menu will appear to choose which kind of file to upload. This will open a window to select which file to upload. The subtitles will be pulled from your .srt file and placed into the timeline and transcript. Edit changes to the text in the transcript or alter timestamps by clicking and dragging the bar on either side a subtitle in the timeline. Press “Publish” and your subtitles will be uploaded to your video.

Summary:
Open a text editor. Create your subtitles. Go to “File” and select “Save As…”. Enter a filename ending in “.srt”. Click the “Save As Type” and select “All Files”. Click the “Encoding” menu and select “UTF-8”. Go to your YouTube “Video Manager”. Click the “Edit” dropdown arrow and select “Subtitles and CC”. Click “Add New Subtitles” and select “Upload a File”. Select “Subtitle File” from the popup menu. Browse for the file you created and select “Upload”. Adjust your subtitles. Publish your video.